I'm going to say John Anderson. He's not going to be of considerable influence or importance after this election. He's just the one establishment Republican this year willing to put his money where his mouth is for #neverTrump. But after whatever resulting fallout 2016 has, he's just going to quietly go back to whatever he was doing prior to this year.
Anderson actually didn't originally intend to fade away quietly. He had planned to start a third party called the National Unity Party, which would run a candidate in 1984. Ultimately though, he did not wish to end up being a spoiler, and he endorsed Mondale. But I do think it's a decent comparison.
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.
This is pretty accurate as well.
