I think this might depend on whether FDR could get Wallace to come clean about the "guru letters." (Supposedly in 1940 the letters never came up because FDR threatened to make an issue of Willkie's relationship with Irita Van Doren
https://en.wikipedia.org/wiki/Irita_Bradford_Van_Doren if he used them. AFAIK, Dewey had done nothing to leave himself vulnerable to that kind of blackmail.) Left to his own devices in 1948, Wallace handled the issue incredibly clumsily, as I wrote in an old soc.history.what-if post:
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There is of course a case to be made that Wallace could simply have said that this happened years ago, that it was not important, etc. The problem is perhaps not so much the letters themselves as the way Wallace would have reacted to their revelation. In OTL, when Westbrook Pegler raised the issue of the letters in some columns in 1947-8, Wallace handled it very poorly at a press conference. To Pegler, who asked if he had written the letters, he said "I never engage in any discussions with Westbrook Pegler." To other reporters who repeated Pegler's question, he said "I will not engage in any discussion with any stooge of Westbrook Pegler." A reporter from the *Washington Post* objected that she was no stooge of Pegler's, that she did not agree with his columns, but that she thought Wallace should say whether he wrote the letters or not. Wallace refused.
Finally, a well-known journalist asked Wallace, "Would you consider me a Pegler stooge?" Everyone, including Wallace, laughed.
"No, Mr. Mencken," Wallace said. "I would never consider you anybody's stooge."
Mencken continued, "Well, then, it's a simple question. We've all written love letters in our youth that would bring a blush later on. There's no shame in it. This is a question that all of us here would like to have answered, so we can move on to weightier things."
"I will handle that in my own way and in my own time."
"But why? These things have no importance."
"Let's get on to something important."
"Why not now?" asked Mencken. "We are all here."
Wallace still would not answer. When Doris Fleeson said "some people defended you and your actions in 1940 and 1944. You owe it to them to clear up this matter," Wallace said he would--but not here.
(David Pietrusza, *1948: Harry Truman's Improbable Victory and the Year That Transformed America,*, p. 257)
If Wallace would handle the issue that badly in 1944, the Democratic ticket would be in serious trouble. I realize that usually the choice of vice-president doesn't affect that many votes, but I believe this could have been an exception (especially given the concerns about FDR's health). And that 1948 was not an aberration for Wallace on this issue is clear--he *never* really owned up to the letters. As his sympathetic biographers, John C. Culver and John Hyde, write in *American Dreamer: The Life and Times of Henry A. Wallace*, p. 134:
"At least some of these mysterious letters were of questionable authenticity. The typewritten letters, for example, contain spelling errors that would have been uncharacteristic of Wallace. Wallace himself did what he could to muddy the question of their authorship. When the letters almost became public during the campaign of 1940, Wallace prepared a statement (never released) flatly denying he wrote them. In 1948, when portions of the letters did become public, Wallace contemptuously refused to talk about them at all.
"Several years later, in the oral history he gave to Columbia University, Wallace offered a carefully hedged explanation of the letters. The so-called guru letters, he said, consisted of 'unsigned, undated notes, which I knew I had never sent to Nicholas Roerich, but there were a few letters addressed to Nicholas Roerich signed by me and dated which were written in rather high-flown language.'
"This was Wallace's only recorded comment on the guru letters, and it was misleading at best. Many of the letters, as Wallace well knew, were not addressed to the guru but to others around him, including Roerich's wife and son and his chief assistant, Frances Grant. The authenticity of the handwritten letters, as Wallace in effect acknowledged, was indisputable. Even the typewritten letters comport, in tone and substance, with other letters of a spiritual nature written by Wallace during the 1920s and 1930s."
http://books.google.com/books?id=dvFxSQVSWm4C&pg=PA134