I don't know how you can exactly get to there but assuming that, on December 7th 1941 (or whenever butterflies has the US join WW2) the US is under a Republican President whereas the UK is under a Labour PM, then what would've happened? As for the specific politician idk, but let's assume that they are both under the mainstream of their parties in the 1940s. Fwiw I think an easy way to get a Republican in charge of the US in the 1940s is to have FDR choose not to run for a third term, and then having a scandal befall the Democratic Party which has the Republican Party win by a slight amount.