They are leaving ahead of the elections next year. I can think of quite a few reasons why they might do that.

• Staying means being forced to either defend the indefensible or facing backlash and challenges from the right, either of which could damage any future political ambitions they may have.
• Getting out now leaves room for others to get elected, which keeps them from being held responsible for their party losing a seat.
• They know how nasty things are going to get and they want out before leaving becomes too dangerous. It wouldn't be the first time that Republicans in congress were afraid that pissing of Trump's base could put them in harm's way. That was not helped by Jim Jordan supporters agitating their followers and stirring up death threats against their colleagues just to get their way during the fight to become speaker.
• Because they are getting pushed out behind the scenes and are choosing not to fight. I'm skeptical of this but I can't say for sure that it isn't happening.
• Because they expect the next election to be a disaster and they don't want to get caught up in it.

Basically, most reasons come down to either just wanting out, or wanting to make sure they don't ruin their future political career choices.

If they stayed and voted with democrats, went independent or switched parties, or tried to start a new party, most likely they would end up just as unemployed but with fewer friends and no followers. Going directly against the party would lead to the party itself attacking them, along with the right wing media that many of their supporters get all their info from. Most districts lean to one side or the other, and while a few politicians have made careers on being independent or moderate, the ones that had been on the Republican side of the aisle are all gone, as are those who had the courage to take a stand against Trump when it mattered.

Honestly, at this point they may actually be able to get more done by quietly coordinating with others who have left or been forced out and organizing support for whoever emerges as a viable Trump alternative in the Republican primary. That keeps them out of the cross hairs and at least increases the chances that Trump will lose either in the primaries or the general election if they can at least drag out the fight for the nomination.

If Trump loses badly, political winds could shift in the party. Or if another politician gets the nomination and goes on to win the election, they'll be in a position to push Trump's people out. That's how his people took control in the first place.

The thing that is different about Congress is that Reps and Senators are free to vote with or against the rest of their party without any repercussions. We don't have three line whips or anything like that here, so the party system isn't as powerful. This is why Manchin could go against the Democratic Party agenda so frequently and the Democratic leadership couldn't do anything about it.

So in theory these Reps could stay Republican and vote with the Democrats, or go independent/libertarian/etc if they really want to make a break with the Republican label. Staying or changing parties really doesn't matter except in how it defines the majority party in each house, and also practically as to what legislation is likely to get brought to a vote per the Hastert Rule or the Standing Rules of the Senate.

So this all can go to explain why they're not changing parties or who they caucus with. It still leaves the question of why they're choosing to leave rather than remain and affect change from within. Answer: They don't care about the country or the party. They were here for power, the lobbying money, the post politics sinecures, etc. Now that all of that is gone, they have decided to cut and run. They are abandoning ship now in the hopes that they can get the best life raft.

We have effectively two rounds of elections. In the primaries we determine who the candidate will be from either party. That candidate then runs in the general election against the other candidate.

Republicans have now run into the problem where party voters in primaries will pick candidates who can't win the general election. The Republicans retiring could possibly win another general election, but they're not extreme enough to win a primary anymore.

Plus Republicans aren't known for courageous behavior. Retiring quietly is what they do.

They are vacating now to try to come back later after the zealots infighting kills them off.

They're resigning their seat but not leaving the party, as I understand it. Makes no sense at all.

I agree, we need a 3rd party terribly right now. You guys have got the edge on us in that.