Why did republicans gain so many seats in the 1942 midterms?

[Orser67]

-45/-8 is basically just a normal mid-term, especially when you consider that the parties essentially split the total number of Senate races won that year, and that Democrats retained control of both houses of Congress. Democrats did probably get a little unlucky in that Operation Torch (the Allied landing in North Africa, and the first major U.S. operation in the European theater) took place pretty much right after the election.

[Orser67]

I wonder how the 1918 midterms might've been different if the armistice had happened just a little bit earlier. Sure second term midterms tend to be bad, but the congressional majorities Republicans won that year weren't huge.

Yeah, that would be interesting, and could potentially have had major implications if Democrats held onto the Senate and possibly managed to push through the Treaty of Versailles (although they would still face a difficult battle in getting the two-thirds vote).

[Orser67]

How much of the Senate losses could be seen as a correction from 1936?

Quite a bit. It was really a broader correction from the Democratic high point of the first half of the 1930s, as Democrats had had an historically unusual run of four straight good even year elections from 1930-1936.