Unfortunately, a significant part of what enabled Willkie to get the GOP nomination was the fact that Roosevelt was going for a third term. Without the clear internationalist stance on the part of the Dems, the corresponding segment of the GOP wouldn't have been able to muster the momentum it did.
My sense is that had the Dems nominated a more conservative standard bearer (Garner? Hull was getting rather up in years, I believe) or more controversial one (Farley, perhaps, even though he was Catholic--or if you really want a stretch, Joseph Kennedy), the GOP would have wound up with Taft or Vandenberg. Both were isolationists at the time; Vandenberg came around shortly thereafter.
Try that on for a dystopian TL: a defeatist / America First type in the form of Kennedy running against an isolationist in the form of Taft. That suggests to me the UK is going to have to go it alone for quite a while against the Luftwaffe for openers. If Pearl Harbor happens on or about the same time, there'll be a showdown in the Pacific--but that had been brewing for 45 years or so anyhow. And that'll go on to end via conventional arms, since it's difficult to imagine either Kennedy or Taft backing Einstein's recommendation for what became the Manhattan Project.
Maybe it's possible-not sure how-that the so-called "amateurs" could put Willkie over the top to face Kennedy, Garner, or Farley. In that case, Willkie succeeds Roosevelt, and probably the course of events isn't greatly different than in OTL. Willkie's internationalism was fairly close to Roosevelt's, so I could see him backing similar measures. The only kicker is what Charles McNary does on assuming the presidency in '44, if Willkie dies then. One would hope he'd be a caretaker in effect for the rest of the Willkie term and then pledge to see things through to conclusion based on Willkie's policies.
All that would make 1948 very interesting. You could have Ike running for the GOP against who knows who for the Dems: not Truman since he'd never have been VP.