1920 was effectively a referendum on the Wilson administration, particularly the foreign policy. Roosevelt was in no means an isolationist, but he was a very popular President. It's a sketchy subject, but I think a GOP victory was inevitable. You'll probably see a isolationist VP on the ticket like Hiram Johnson (which would be his exact ticket from the Progressive Party in 1912).
Okay, so what if Johnson heads the ticket, with TR backing out of the race, endorsing the Boy Governor? Who does he pick as VP, and does this lead to a conservative Democratic nominee?