Who knows. Huey's plan was to run against FDR in the 1936 primary to popularize his message, and then in 1940 break off and form a third party.
A third party in 1940 might've meant a Wilkie presidency.
Iirc long's plan was to form a third party in 1936, but have a fellow progressive senator run on the ticket. Long would campaign heavily for his party and hopefully allow Landon to win the election. This would prove long's ability as a national vote getter to the various Democratic machines while allowing him to atleast partially absolve himself of the blame for being a spoiler. Then in 1940, Long would agree to fold his party if he was granted the democratic nomination.
I don't think this would've been effective however. FDR was extremely popular. Long was popular too, but he lacked the profile that FDR had. And he would've been partially relying on a separate, less charismatic figure to syphon votes from roosevelts left. Even if we grant him a victory in 1936(in the form of a Landon presidency), I very much doubt he could've won the nomination in 1940. Most factions of the Democratic Party apparatus disliked Long, a lot. Sabotaging Roosevelt wouldn't have changed that.