I'm not entirely convinced the U.S falls for the bait and declares war on Hitler and Mussolini under Wilikie. We were attacked by Japan, and a Wilikie administration would focus on them.
Most likely, we go to war in the Pacific, Europe remains largely an undeclared war for the U.S until Japan is defeated (at least in 1945 when the nukes are ready, possibly sooner given there are no American troops in Europe), followed by a quick pivot to Europe and declaration of war, just in time to knock out Germany and Italy in one fell swoop in mid-to-late 1945-early-1946.
As for Russia, Lend-Lease continues as is, with no real changes, and probably ramped up with the formal absence of American troops in Europe for both Britain and the Soviet Union.
The one-step-at-a-time strategy seems logical on the surface but:
1. Hitler declared war on us and may have been in a position to actually bomb across the Atlantic if we mostly ignored him for multiple years.
2. Building on #1, if the US focused almost exclusively on Japan and D-Day is delayed multiple years.
there's a very serious risk that Germany would have also developed nuclear weapons by the time the Allies are ready to liberate Western Europe. The US would end up in a Cold War with Nazi Europe, which is obviously worse than the actual outcome of WWII.
You're assuming that the Soviet Union wouldn't be able to win back Eastern Europe and push on to Berlin regardless. It might take longer since there are no other fronts, but assuming Lend-Lease is still a thing (and Wilikie almost certainly would have kept it),they'd have the materials, and they already had a manpower advantage on the Nazis.
As for the Atomic Bomb, there was never any real danger of the Nazis getting it, because of Hitler's obsessions with pseudoscience and determination to put the Holocaust above everything else.
