As @Mumby noted, I had it happen in Decades of Darkness. That was due to a rather special set of circumstances, though.Has anyone ever had the main dominions being split off to separate but unified thrones? I don't have the knowledge to do it
Presidents of the NewStates of America:
1955-1964: Rexford Tugwell (Democratic) 
1964-1973: Orville Freeman (Democratic, Labor after 1967) 
1973-1976: Milton Friedman (Liberal Republican) 
1976-1985: Edmund Muskie (Labor) 
1985-1988: Ronald Reagan (National Unionist) 
1988-1997: Ron Dellums (Rainbow 88, DSA after 91)
1997-2000: Joyce Brabner (DSA)
2000-2009: Tom Ridge (Liberal Republican) 
2009-2018: Ted Turner (Labor-Progressive) 
2018-0000: Joe Biden (Liberal Republican) 
(Just to clarify the electoral system: an election happens every 9 years, but the president's government faces a referendum after 3 and if they receive <40% then an immediate election gets called. Also there's two VPs)
 FDR's deputy, the architect of the Second New Deal and the man behind the Allies' victory in the Second World War, was perhaps the only man capable of so radically altering America's postwar political landscape. After the NewStates Convention was finally ratified by the requisite two-thirds of American states, there was only really one obvious candidate to be the NSA's first president. President Tugwell would triumph over scattered opposition from renegade Democrats and a GOP that was still very much stuck in the wilderness.
Tugwell's presidency mainly dealt with the difficulties in trying to establish his new government's legitimacy. Tugwell's push for racial equality under the NSA's new constitution received violent and sustained blowback from conservative southern whites. Racial tensions between blacks and whites in the American South would remain tense throughout Tugwell's term and after it. In western states, many not in favor of Tugwell's economic central planning, there was heightened resistance to the actions of Tugwell's executive branch and those of the planning branch of government. Although Tugwell would be able to assert the NSA's authority out west, hard political fault-lines would be drawn in regional politics.
Besides 1959's 'Little Crash' and its subsequent brief recession, the economy was largely able to stabilize around the government's new Planning Branch and its National Planning Board. The latter part of Tugwell's term would revolve around foreign affairs, however. The fall of Cuba in 1961 to the communist forces of Fidel Castro would make the Cold War a pressing issue to the American public and something that Tugwell would have to deal with immediately. In 1962, the NewStates would join the Western Union Defense Organization (WUDO) then made up of capitalist western Europe and Canada. Standoff between the two superpowers in East Asia would bring the world to the brink of nuclear war in 1963 but Adlai Stevenson, Tugwell's VP for Foreign Affairs, would be able to bring the Americans and the Soviets to the negotiating table.
As Tugwell declined interest in a second term in 1964, a flurry of government supporters and opponents raced to succeed him. However, only one would come out on top...
 Coming from the Left of Tugwell's new political order, Freeman came to the Presidency offering to work with the regional governments much as he had done in his native Minnesota which gave the Midwest and California the push that got him into office. Despite this, Freeman immediately went to war with the Southern states over racial equality - mainly that African-Americans were entitled to the same share of new welfare programs as white Americans. From there the rest is history, as the economic struggle became political, Freeman came down hard on the South determined to use Federal power to break the back of Jim Crow legislation.
All of this did nothing for the political unity of the Democratic Party. Many of the Southern Democrats began to leave the Party to establish their own, while moderates in the West began crossing over to the Republicans. Figuring to go his own way, Freeman took the Left of the Democrats to establish his own party - the American Labor Party from the offset mirrored its European counterparts, with its roots in Trade unionism, social democracy and support for minorities. Nevertheless, Freeman's success in the 1967 referendum usually credited only to his advantage as the sitting President.
Being able to stay on by the skin of his teeth, Freeman took a much more moderate approach: the Universal Healthcare Act was scaled back, increases in welfare had to be reduced and the plans to give Unions a bigger voice in the National Planning Board. Like his predecessor, Freeman's latter term became engulfed in foreign affairs, a freedom only granted as the economy ticked up and up. After another Latin American Republic fell to Communism, Freeman took the idea that the Monroe Doctrine applied to Soviet Imperialism as much as European. As Argentina looked to turn a quarrel over Tierra del Fuego to all out war with Chile, the NSA swooped in to the rescue. The sudden arrival of a carrier group off River Plate and an 82nd Airborne detachment sent to troop the colours in Santiago changed the drum beat coming out of Buenos Aries. This became the NSA's first offensive action in the hitherto defensive Cold War.
For a time, Freeman actively considered running for a second term in office, however initials polls saw his name drag that of his new party down. Hoping to save his new Party from an electoral wipe out, Freeman refused to accept nomination - sponsoring instead one of his VPs, Bayard Rustin.
 Friedman's victory was due less to him being a charismatic speaker or his ideas being liked by many, it was mainly due to the ALP and Democratic Party taking chunks out of each from the Left allowing the new Liberal Republicans to win by a small margin. The Liberal Republicans were a project created and run by Friedman to help spread the idea of monetrism, privatisation and deregulation. Which is what Friedman tried to do.
Instantly there were problems, the Labour Unions buoyed by many years of support and help weren't particularly fond of the idea of privatisation or them being left out just yet. Cue numerous strikes and worker actions which Friedman's responses of trying to have the police crush them often failing or turning the press against him. His military actions in South America ended up leading to a quagmire occurring in Peru against the Communists guerrillas and his lack of caring when it came to things like the rise in drug use (Friedman would allow for the legalisation of Marijuana in 1974) or Gay Rights (which he would support) caused more Conservative voices to cry out for blood. In the end though it would be his monetary policies that would lead to his downfall. The deregulation of the financial industry combined with a crash in oil prices due to the 1973 Gulf War lead to a recession in 1974, with Friedman's lack of action against it causing much resentment. When the Referendum came in with Friedman only gaining 33% an election would be called and Friedman hoping to keep his party afloat stepped down to pursue being an economics adviser for the Singapore Government.
 One of Freeman's powerhouses in the Senate and a bitter opponent of Friedman, the 'Man from Maine' came to office promising a return to regulations, the protection of the environment, adapting welfare to fit the challenges of the 70s, and detente in the Cold War. The American public liked most of this. Muskie's problem was they weren't as sure about his actual party, and Muskie found he had a minority in Congress and could only get his aims through by compromises with other parties. Instead of a formal alliance, he would carry out the Muskie Doctrine (as it was called) of personally wooing Democratic, Liberal Republican, and independent & 'local' congressmen (the latter a result of Tugman's changes and the political discourse making it easier for a state-specific candidate or party to get at least one man in) who might come round to his way of thinking.
It was successful, but it was slow. Changes came in domestically and he achieved much of what he wanted, but the pace meant this went unnoticed by many and his welfare improvements were two years behind the times when they were completed. It was not until years later he would get his due, instead seen as not meeting his promise. He almost lost his second referendum.
In terms of foreign policy, however, he was able to pull off a global reduction in nuclear weapons. He also used the US's soft talk and big stick to reduce tensions in South America, allowing "homegrown" socialist states and discouraging the brutalisation of activists by right-wing forces. He would be renowned globally for this peaceful use of power. (After his death, it would come out he'd also used the CIA and NSA to sabotage a few political careers 'down south' if those politicians were considered a threat to keeping everything on an even keel)
 The National Unionists were a bastard mockery of Abraham Lincoln's electoral vehicle - being a fusion of white supremacist Democrats and moralist Republicans. And like the National Union of old, they were essentially a personality vehicle. Reagan had begun his political career as a Democrat, even an enthusiast for the Newstates Constitution. But by the 1980s he was the very incarnation of all the frustrations that had been building for nearly forty years.
Reagan was a force of nature, achieving victory essentially thanks to his celebrity status. His actual agenda was virtually unknown.
That his agenda essentially looked to abolish an entire branch of constitutional government, take Friedmanite economics to a new height while rolling back decades of social and racial progress, not to mention taking the Cold War to a grand new height in South America. It should therefore come as no surprise that in 1986 the National Unionists were essentially purged from office, amidst a Planning Branch crippled by cuts and the worst unemployment crisis since the Great Depression. Anti-war protestors linked hands with the unemployed and with sexual and racial minorities. Reagan had essentially created the movement that would remove from office by referendum.
 America wanted a brighter future, it wanted Progressive ideals, Environmental efforts, Social Justice and to get out of the worst unemployment crisis in history. The Rainbow 88 coalition offered that. With Ron Dellums, the Charismatic Democratic Socialist from California being there leader this coalition of Labor, Progressive Democrats, Socialists, Marxists, Communists, Trots, Liberals etc. Would sweep into office on a popular mandate.
Dellums offered America a lot and his coalition would try and bring it,The Planning Branch was revived, New Deal style projects were implemented to help unemployment and expand environmental friendly power, Welfare increased, Housing increased, the state expanded to greater lengths than ever before.
Of course unlike much of the previous efforts by the Planning Branch, Worker owned Factories and CoOps would be supported to and attempts to establish a Market Socialist system implemented. Civil Rights would be expanded as well with greater emphasis on helping all races, genders and sexualities due to the horrors of Regan administration, though projects like Same Sex Marriage would only start to gain light towards the end of his presidency.
Dellums would also be an Internationalist, putting pressure on South Africa to end Apartheid by 1990 and would help ease the Liberalising Warsaw Pact into the so called ‘Market Socialist Sphere’ and would also begin to climb down from the interfering in South America and help a number of democratic elections occur there. When the Referendum came Dellums would pass with a strong 74% showing that this Democratic Socialist had done much positive change with many considering him to be one of the best Presidents. When he finally left office America whilst not in that bright future as promised was still much bright than before and could head into the 21st Century with optimism.
 A political activist and independent comic creator, Brabner had joined the Cleveland wing of the 'rainbow' protests against Reagan and by sheer force of will was able to to take a commanding position. She would become nationally famous for the series of journalistic comics about Reagan-era Pittsburgh with her husband, Harvey Pekar, and this propelled her into the House as a Rainbow '88 and later DSA congresswoman. Under Dellums, she would work on prison reforms.
Thanks to her combatitive nature, strong will, fame, and impeccable ideological credentials, she won the vicious fight to succeed Dellums and so the first female president followed the first black one. Once in office, she worked to further the DSA's transformation of the state and continue the eviseration of the intelligence community. She expected a pushback from that. Unfortunately, the bigger problem was Brabner's abrasive personality - fine as a planner and administrator, and something that was fine in TV debates with rival politicians, but it made her one of the least media friendly presidents. Her struggle with depression would also flare up and became public knowledge. This, along with the DSA being in power long enough to now become the establishment norm (it had became a point of concern for years that the youngest voters saw other parties as the cool rebel ones), would lead to her losing the first referendum on her presidency.
 It was not lost on American Conservatism that each time a figure from the Right had entered office that he had never made it past his first referendum. While unpalatable it had to be accepted that Socialist Economics and Progressive Social reform had the broad support of most of America and attempting to rollback on these would simply lead to another eventual defeat. Instead they battled the election on two fronts they felt most confidant on; Law and Order and Foreign Affairs.
Tom Ridge was elected as the first Liberal Republican in 24 years promising a crackdown on perceived inner city lawlessness (a subtle dog-whistle to the more unreconstructed National Unionists) and an increased push for international Democracy, led by the NSA. There would be some tinkering at the edges of the Market Socialist system, some privatization of non-essential services and some minor tax restructuring but nothing like the previous Monetarist reforms. Ridge kept the Unions onside by keeping a portion of the Board of the new privatized companies open for workers representation and a promise to not renege on labour protections. Ridge also promised to respect standing LGTB laws and equality provisions as settled law albeit with the unspoken caveat that the LR's would not be championing any further large scale changes.
Internationally Ridge would seek to work with the Soviets and Chinese to combat the rising tide of Islamism within Central Asia, brought into stark focus by the Victory Day Moscow bombings which left 350 dead during the parades. America would share intelligence and provide some special forces assets to hunt down militant leaders on the Tajik/Afghan border, however there would be points of contention with their erstwhile allies when the Soviets made some moves to encroach on the Northern borders of Imperial Iran ensuring that relations remained somewhat frosty.
Riding high on foreign successes and a strong economy (albeit one instigated by the previous administration) Ridge easily won his referendum, the first Conservative to do so. His following 6 years would remain calm and relatively uncontroversial barring an outcry when his law and order policies were adjudged to be focused overbearingly on urban ethnic minority groups. However with his support base being primarily rural and White he was able to ride out that particular storm.
Come the end of his 9 year term he declined to seek another term, citing precedent and his success in finally being the first 9 year President from the Right. With America prosperous and the world situation fairly stable it seemed the perfect time to pass the fruits of his labour to his successor.
 It says a lot about Ted Turner that despite being a Millionaire in partial control of a multimedia empire he lead a political party full of Progressives, Social Democrats, Eco-Socialists, Trade Unionists and Market Socialists with a name taken from a Canadian Far Left party. Turner had originally been a Republican but after there collapse during the 60s and the rise of Freidman's Liberal Republicans he turned away from them and started to become friendly with Freeman and especially the Muskie administration. When Reagan had been elected, Turner shifted his aim at the administration and his constant attacks on Reagan as well as funding of the Rainbow 88' Coalition. Turner would support the Dellums and Bradber administrations and would allow partial public ownership of his media networks and would support the actions of the Turner New Union. As Ridge and the Liberal Republicans ruled America, Turner would start attacking Ridge for his lack of support for environmental concerns, avoiding creating a Green economy, not supporting full scale progressive changes and for targeting minorities.
When the 2009 election reared, Turner turned to the old Labor party (still limbing in the background as the DSA took most of the Left votes) as well as the remnants of the Progressive Democrats/New Dealers/Tugwellites and Rainbow 88' coalition and create the Labor-Progressive Party as his election vehicle. Whilst the Liberal Republicans entered into a slugging match with the DSA, Turner's populist appeal and progressive values would win out and he would manage to enter into office. His aims for his administration were improving the inner cities, environmental reform, a Green Economy and social change. With the aim to make America Carbon Neutral by 2020, Turner would manage to get the Unions related to Fossil Fuels on board through his grand 'Green New Deal' using the Planning Branch to help sort out jobs in coding, computers and media leading to the creation to the Silicon Belt in America. He would also expand natural parks, reintroduce animals like Bison and Wetslope Cutthroat Trout to America, ban Gas and Oil excavation (the Liberal Republicans and Fossil Fuel Companies would cry foul but Turner would shovel money in there direction to shut them up). Social Change would be increased with many LGTB laws being expanded to include more protections for Bisexual,Trans and Non-Binary individuals.
Turner would manage to pass his referendum with popular support but after that he started pushing some odder ideas, attempts to implement a two child policy would end up being rather controversial, his bizarre antagonistic relationship with America's Christian community, Turner would complete the complete destruction of the N.S.A's Nuclear, Chemical and Biological Weapons (leading to many folks crying he was leaving America naked at the negotiation table), his support of Palestine over Israel on certain issues would cause controversy amongst elements of Americas Jewish population and his pal like relation with the leaders of the Soviet Union, China and North Korea would lead to the Chairman of the DSA Tim Ashe calling him out for 'Being Friends with Dictators' in an infamous speech. Still Turner would be seen as a fairly popular president amongst the Left and Centre and when he stepped down in 2018 few could say he had changed America.
 While the Labor-Progressives sank like a stone without Turner, it wasn't fast enough for the DSA who despite performing well downballot, failed to shrug off Ted Turner's body blow to their presidential aspirations. The path was opened up for the Liberal Republicans, now firmly ensconced as America's Party of the Right.
Biden had once been a National Unionist, on the softer edge of the Democrats who formed up behind Reagan. And it showed. On any given subject, if you dug into Biden's past there was a typically Reaganesque soundbite, whether that was about drug laws, gay rights, racial integration, taxation or social security. But during the decade of DSA rule, Biden moved away from the National Unionists and after a period sitting as an Independent, joined up with the post-Friedmanite Liberal Republicans.
The narrowness of Biden's victory in 2018 is a cause of some controversy - not least because the elderly leader resembles Reagan in more than just his politics. His speeches have become increasingly inflammatory and incoherent in recent months as the date of the Presidential Referendum approaches. And not only that, he appears to have done things even Reagan was never accused of. Allegations of sexual misconduct have flown thick and fast since his inauguration and has led to the international sensation of the ThatCounts movement, dredging up decades of sexual impropriety and assault from across the political and business worlds. The downfall of the USSR's stodgy conservative elite to be displaced by an ambitious group of anarcho-syndicalist feminists dubbed by the typically misogynist Morning In America News 'Pussy Riot' (no matter how often they say thats a literal translation of the group's name, they've not really recovered from that).
Despite this, the prospect of him losing his referendum next year is not a certainty. The President's erratic charisma, his right-wing record and association with the National Unionists of old, and his recent tendency to not so much dogwhistle as play a brass band all on his own, had endeared him to an aggressive new generation of right-wingers. With the left more divided than it has been since before Reagan, there is every possibility that Biden may yet clinch victory.