Answer:
The United States Populist Party was active in the last decade of the 19th century and the first of the 20th.
Factors:
Factor 1: There was a polarization within the party when two trends are clearly expressed: the "fusionists” , who wanted to ally themselves with the Democratic Party, and the "mid roaders" who wanted the Populist Party to be a third independent party.
Factor 2: The party weakened a lot when the Democratic Party candidate William Jennings Bryan was not elected, with which the Populist Party had allied, for the US presidential elections of 1896. Jennings Bryan did not get people from urban areas or industrialists vote for him. He obtained 47% of the popular votes thanks to the rural areas of the South and the West. This fact and the defeat of Bryan buried the dream of forming a third populist party that brings together farmers and workers.
I consider that the determining factor was the alliance with the Democratic Party since it did not integrate the feeling of the people who wanted to group under a populist ideology since these militants did not find coherence in merging with a party that accepted and promoted visions that favored the interests of the elites and powerful merchants and industrialists.