Some Events...
The 107th Congress is often remembered as the "Disaster Congress", as numerous resignations due to illness/injury concerns following UA101's collision with the White House, party switches, and special elections caused heavy amounts of chaos to occur.
Resignations/Vacancies Caused By Capitol Hill Attacks and Surrounding Environmental Effects:
Don Young (R-AK-AL), seat taken by Tony Knowles (D)
Patsy Mink (D-HI-02), seat taken by Matt Matsunaga (D)
Floyd Spence* (R-SC-02), seat taken by Joe Grimaud (R)
Dave Obey (D-WI-07), seat taken by Gary Sherman (D)
Patrick Leahy (D-VT), seat taken by Bernie Sanders (I)
Following the May 1st Attacks, many Republicans became outspoken critics of President Andrews or of the Republican Party, leading to either party switching or harsh criticism, those who engaged in such activity in the U.S. Senate are listed here.
John McCain (R)
McCain was opposed to some of President Andrew's more conservative social policies, but he remained a Republican and became a major supporter of the War on Terror
Ben Nighthorse Campbell (R) -> Ben Nighthorse Campbell (I)
Unlike most Republican critics of Andrews, Campbell changed parties not over the War on Terror or social policies, but due to his nomination of Pat Miller as Secretary of the Interior in 2002, who Campbell heavily opposed. Campbell had long distrusted Andrews, and did not support him in the 2000 election, instead reportedly voting for Reform nominee John Hagelin.
Olympia Snowe (R)
Snowe remained a harsh critic of President Andrew's social beliefs, but remained a Republican.
Bill Weld (R)
Weld was heavily considered by Democrats as a potential convert to their party, but he remained a Republican. However, Weld's outspoken opposition to many of Andrews' policies lead to him unsuccessfully challenging him for president in 2004.
Gary Johnson (R) -> Gary Johnson (I) -> Gary Johnson (L)
Johnson's commitment to non-interventionism and opposition to the War on Drugs caused him to be frustrated with the Andrews administration, and he became an Independent. Shortly after the 2002 elections he changed his registration to Libertarian, and mulled a run in 2004 or 2008 before ultimately endorsing Ben Nighthorse Campbell in the 2004 general election, angering many Libertarians.
Norma Paulus (R)
Similarly to Weld, Paulus became a loud critic of the Andrews administration, but due to her re-election prospects she remained a Republican, something that killed her in the Republican primary in 2002. Paulus would later join the Democrats and support Ted Turner in 2004.
Lincoln Chafee (R) -> Lincoln Chafee (RII)
Chafee was one of the only Republicans to openly oppose war prospects, and decided to form the Rhode Island Independent Party with Bob Healey, and spent much of the rest of his term focused on local political issues.
Jim Jeffords (R) -> Jim Jeffords (I)
Jim Jeffords had long felt disconnected from the national Republican Party, and reportedly planned splitting in early 2001. He would do so later in the year, and following the election of Bernie Sanders in December 2001, Vermont would have an all-Independent delegation.
On the other side(s) of the aisle, the exact opposite happened, with senators
Glenn Browder (D-AL), Zell Miller (D-GA), and Wes Watkins (I-OK), joining the Republican Party over some Democrats non-interventionism and liberalism on social issues.
Final Senate Composition [107th Congress]:
Republicans: 65 Senators
Democrats: 31 Senators
Independents: 2 Senators
Libertarians: 1 Senator
Rhode Island Independents: 1 Senator