In this situation, a lot of Americans would be tempted to take Truman's advice: support the Germans if it looked like the Russians were winning and support the Russians if it looked like the Germans were winning, and thereby cause Hitler and Stalin to bleed one another white.
This said, hard to believe that with the Red Army 'liberating' Warsaw and the Brits relieved of air attacks, the U.S. would start supporting the Nazis. They could well stay out of the war, but I'm not seeing an invasion of Canada!