What did the Populist Party believe in? And what did the Populist Party NOT support?
They criticized railway companies trusts and the banking power enabled by the gold standard.
At the end of the nineteenth century farmers in the Midwest were very critical of the trusts and of the gold standard. The Gold Standard made inflation completely impossible. Only rich bankers could afford to have gold.
Gold Standard created deflation which lowered the benefits of farmers. Therefore it was harder and harder to pay back their original loan. If you borrow 1000 $ and you make less and less it is obvious that paying back is going to be harder. The railway companies trusts were able to impose very high prices.
The Populist Movement thus rose in opposition to that hegemony. William Jennings Bryan, a Nebraska lawyer with his "Cross of Gold" was a well-known critic of the gold standard who ran in the 1896 presidential election, one of the most crucial in American History.