There are still plenty of Republicans, both in office and in the rank & file, who are vocally opposed to President Trump. Fighting from within is the only good option short of splitting the party in two.
1) Who cares about splitting the party in two when both sides are going down with the ship? You're losing states in the reddest states in the union. You're on the Titanic. Who cares about preserving the hull or whatever?
2) Unless tragedy befalls him, the only people who can do anything about President Trump is the Republican Party. Fifty years from now, what chapter would you rather read? "Under President Trump, Republicans suffered large defeats in both houses..." or "In an unprecedented move, the Republican party stood up to President Trump and moved to impeach him, costing them a base of support, but they were able to regroup under President Pence and rally behind him in his bid for reelection?" What would make you proud?
3) I want to go back to "splitting the party in two." I understand why in some ways that's bad for your party. But every day you keep this ship afloat, more Holocaust deniers and race-baiters jump aboard. This is only happening with the Republican party. More and more white supremacists look at the Republican party and like what it has to offer them. I want to underline that last sentence because whatever words that your elected Republicans are saying against Trump's message isn't being heard by the the worst people our country has to offer. In your heart, do you honestly think once Trump is out, the party will go back to the simpler, more moderate days of...The Tea Party? The neo-cons?
I want you to ask yourself something: who will run for office in your party after all the Holocaust deniers have their shot? What comes next?
4) Finally, on a personal note, I'm a Jew. My grandmother is a Holocaust survivor. Do you think there's a chance I would remain in the Democratic Party if half of them supported an anti-Semitic candidate? If a third of them did? Or if they stood behind a Holocaust denier? We all have a line we cannot cross with the rank and file. Where's yours, dude? When do you say "This party isn't for me anymore"?