Sinclair, but I don't understand why Burton K. Wheeler was dropped.
Roosevelt got more mentions in the convention thread (1) than Wheeler (0), so she got the nomination. I suppose the in-universe explanation would be that, in light of the American Midterm victory in 1926, the Populists felt they needed a fresh face on the ticket to secure Sinclair's reelection.
Despair not: Wheeler doesn't die until 1975, so there's plenty of time for a comeback.
ROOSEVELT/Wheeler intraparty unity ticket somewhere down the line?