Agree, but if he stays in the Senate race and loses, he forfeits his congressional seat.
True, but if the party wants him to resign and dangles the possibility that he can run for his current seat then he may go for that. And in order to do that they would need to move Ann Wagner to the senate race. By all indications she's a solid conservative, at least according to her congressional website. She's a woman, which would make it hard for McCaskill to user the DNC 'war on women' arguments. She's been head of the GOP in Missouri. And she's someone McCaskill isn't prepared to take on. We could do a lot worse.
If he decides to stay in his support will return. The Republicans need the seat to take the Senate.