The GOP laid an egg in nominating a neophyte like Willkie. They should have picked Senator Robert A. Taft (R-Ohio), a lifelong, principled Republican who has done much to revitalize the Senate Republicans, who were nearly Moribund at the start of 1939.
The issue in 1940 -- the only real issue -- was isolation versus intervention.
Taft, Vandenberg et al were strong isolationists, while Wilkie was for providing more US aid to the allies.
Yes, as the fall campaign developed, in typical American fashion, both sides tried to appeal to all voters.
Eventually Wilkie accused Roosevelt of a secret plan to lead America to war.
FDR in tern suddenly discovered isolationism and promised he would "not send American boys into any foreign wars."
When war came in December 1941, all the Republican isolationists supported it.