I once observed that Taft could have the backing (unofficial of course) of the Communist Party:
***
Of course they could never officially endorse him, but maybe just as in 1936 they made it clear that they thought FDR was preferable to the "Hearst-Liberty League-Landon" forces, so in 1940 they make it clear that Taft (if nominated by the GOP) is preferable to the "warmonger" FDR?
As Earl Browder wrote,
"It is not only the Communists who are being disfranchised in 1940. The New Deal masses and the labor movement are left politically homeless. The Republicans are worse off than we, for they have not even a candidate to fight for; they must either vote for a Democrat, or turn to the Communists. The leading and natural candidate of the Republican Party was Robert A. Taft. But he was defeated in the Philadelphia Convention, and the pro-war, big business, renegade Democrat Wendell Willkie, was nominated by a conspiratorial junta, organized by Thomas W. Lamont of the firm of J. P. Morgan, working in direct agreement with Roosevelt, and engineered by Walter Lippmann. Willkie was chosen for the Republican Party by Roosevelt and Lamont, after an agreement had been reached as to fundamental policy to which all would adhere, the same policy revealed in the President's sensational coup of September 3. [Browder is referring to the destroyers-for-bases deal.]
"Willkie's nomination was the guarantee which Roosevelt required before he dared to launch his coup d'etat. The masses, the majority of the voters, had to be disarmed, denied every opportunity of effective protest, before the President dared to proclaim the joining of the United States into the British Empire. Even now, with the elections effectively blocked off from the people, Roosevelt dares not submit his secretly matured plans to a vote of Congress, but must act by proclamation, by edict, by a coup d'etat...
"Imagine, for one moment, what would be the reactions of the great body of American voters to the Roosevelt coup, if the Republican Party had nominated as its candidate Senator Robert A. Taft, that old-fashioned conservative Republican who voted against the Conscription Law! Can anyone doubt that the result would have been such a Republican landslide that it would have wrecked the Democratic Party for all time?"
http://www.unz.org/Pub/Communist-1940oct-00884
(Arguably, the CP may have softened its position even toward Willkie by the end of the campaign. "Curiously enough, four days before [John L.] Lewis made his speech [endorsing Willkie], the Daily Worker carried an advertisement from the Republican-sponsored 'National Committee to Uphold Constitutional Government' opposing the third term. When the Democrats began making political capital out of this apparent collaboration between Republicans and Communists, Browder publicly denied that the CP was calling for votes for Willkie. The advertisement, he insisted, was simply a question of business policy: 'Since the [Daily Worker] is no longer the official organ of the Communist Party, it was clearly within the province of the management to accept advertising if it saw fit.' Whatever one thinks of Browder's assertions about the autonomy of the Daily Worker, it is true that the paper was strapped for funds, and may well have accepted the advertisement for financial reasons. Nevertheless, it is hard to imagine the paper accepting a pro-Roosevelt advertisement in 1940. The incident raises the question of whether the Communists might not have welcomed a Willkie victory in the election..." Maurice Isserman, *Which Side Were You On? The American Communist Party in the Second World War*, p. 269.
https://books.google.com/books?id=iWMprgS8q0AC&pg=PA269)
***
Despite Browder's analysis, I'd have to say that FDR would defeat Taft if anything more decisively. After all, in the end most isolationists (or "non-interventionists") did vote for Willkie as the lesser evil as did most conservatives. So Taft would not too greatly improve the GOP's showing among these groups, while losing some Willkie supporters among moderates and internationalists.