John Palmer, 1896. He ran as a "Gold Democrat" against William Jennings Bryan (who supported bimetallism) and won about 1% of the vote. Bryan didn't have the personal scandals of Trump, but he did represent a major ideological change for the party, and Bryant's intraparty opponents were motivated enough to run a third party candidate after Bryan won the Democratic nomination.
Palmer didn't have the sort of focused appeal that McMullin seems to have in Utah and a couple surrounding states. But the split among Democrats in 1896 was a bit like the split among Republicans today.
