Reagan switched because of the Warner Bros. wildcat strike in 1946, where there was violence, the police and private strikebreakers took care of it, and threats were made on Reagan's life.
Perhaps if the Republicans abandoned Hoover and nominated FDR...though that would predate the PoD. (Hey, Henry Agard Wallace was a Republican for a time...and his father was a Republican Cabinet Secretary!)
Yowza! The guy who's accused of being a Commie/Pinko/Soviet sympatizer, and who gives most of Europe to the Soviets in any TL where he becomes president. This guy was a Republican!? Boy, have things changed!!
have nelson rockefeller republicans take over the republican party,Regan switches the the democrats.
One earlier option is that teddy roosevelt wins a 3rd term as a republican and his fellow progressives take over the republican party. (this is before 1945) sorry, but it was possible