I thick Marshall would be much more willing to compromise and he Coukd get the treaty ratified.
Iirc he said privately that the Lodge Reservations would have to be accepted. If he follows through on that, the ToV is ratified, the Allies probably accept the changes wityh mild griumbling, and there's a US delegate at Geneva, for whatever difference it makes.
Not so sure about after that. Given the levels of postwar discontent, I can't see the Democrats actually
winning in 1920, but they might do better, perhaps holding the Reublicans to 52-42 or something similar, and leaving themselves a chance to recapture the HoR in 1922. They can't make such a big thing of returning to normalcy, if the Marshall Administration has already largely done so. But a lot depends on how the Prohibition and Klan issues pan out.