Had Long survived, I don't see any realistic chance of his winning the presidency in 1936. What bothered FDR was not the prospect of Long actually winning but of his getting four million votes--about one-tenth of the electorate. That was the figure given by a secret poll commissioned by Jim Farley in early 1935. Of course we now know that even had Long gotten that many votes it couldn't possibly have changed the outcome, FDR having defeated Landon by over eleven million votes. (For that matter, four million votes for Long couldn't even have prevented FDR's victory over Willkie in 1940, even if we make the unrealistic assumption that Long's votes would all come at the expense of FDR.) But FDR had no way of knowing that in 1935; many people expected the 1936 race to be close. And FDR even suspected that Long, despite his rhetoric about redistribution of wealth, was in a secret alliance with the conservative Liberty League to bring about a Republican victory in 1936.
In any event, it is very doubtful that Long could even win those four million votes. In general, third party candidates do worse in actual elections than in polls, especially polls taken several months before the election. (The "novelty factor" wears off and the "it's a wasted vote because he can't win" argument becomes more widely accepted as Election Day approaches.) Furthermore, in this case, there is an additional reason to expect Long's support to decline--the economy improved considerably between the time Farley's poll was taken in 1935 and Election Day of 1936. ("The U.S. recovery from the Great Depression was nearly as exceptional as the Depression itself. After falling 27 percent between 1929 and 1933, real GDP rose by 43 percent between 1933 and 1937. Indeed, the economy grew more rapidly between 1933 and 1937 than it has during any other four year peacetime period since at least 1869.1
The most rapid growth came in 1936, when real GDP grew 13.1 percent and the unemployment rate fell 4.4 percentage points. [emphasis added--DT]
https://pdfs.semanticscholar.org/5fe6/9df4347b6492d8ed108401b4078897350bf5.pdf)
Anyway, after Long would lose in 1936, he would be much more likely to end up in jail than in the White House, given the indictments and convictions of so many Long cronies in OTL--some of whom would probably be all too glad to testify against him for lighter sentences. (I know that Long's defenders say that if he had lived, he would have kept them honest. Let's just say that I'm skeptical.)
https://en.wikipedia.org/wiki/Richard_W._Leche
