Two days after Pearl Harbor Germany honored its bilateral pact with Japan and declared war on the U.S. While our declaration of war with Germany was a foregone certainty, the Hitler government preempted our Congress and took the initiative. Thus, the speculations here are meaningless. It was not the predecessors of today's Democratic left who vigorously opposed our entry into WW II, rather, it was those today who would be a part of the religious and policy far right of the GOP.
The nature and perspective of issue oriented wings of the two parties are not static, they have changed over time to occupy the position once filled by the other. It's just as the Republican and Democratic parties have changed their orientation at least twice since the 1880s.