In OTL, the Populists kind of died away as an independent party after they fused with the Dems. I'm not so sure about what happened to their movement, but it was kind of adopted by the urban Progressives who came to occupy both parties, especially the Republicans for a few decades. Woodrow Wilson was a progressive as well, but of the Democrat stripe. In any case, it kind of died down for a while, La Follette's third party run notwithstanding, until FDR and the New Deal and etc.
How could we change it so the Democrats kept the Populist stripe, so by the time FDR came on the scene, he was more populist than the urban reformer patrician progressivism he embodied?