The republicans are afraid Trump will win. Which means they lose. Yet they’re afraid shutting down the government will cause them to lose. It’s their actions that will cause them to lose.
Lots of people fear a Trump victory, and openly hate him. I have come to the conclusion that in order for them to feel that way, they also have to at some level resent or disrespect those who feel Trump is one of the few people running who can restore their voice in America. When Obama made his comment about people clinging to their guns and bibles, he was channeling the disrespect and often outright derisive hatred (or disdain) that many of his (and Hillary's) supporters have for those whose beliefs are different that their own. The irony is that these people actually believe they are the ones who are tolerant and inclusive and enlightened. They are anything but. The Republicans who dislike Trump are a mix of those who are afraid he will win and take away their hold on power via their stature within the Republican establishment, those who profit from illegal immigration - or who are beholding to campaign donors who do, those who are from the old money Republican country club crowd who think he's too crude - and who think that it would be better to lose the election than to be 'crude' like him, and those who resent that someone from outside their ranks could be successfully competing against them. There are likely lots of additional reasons, but I would wager that these cover a fair number of his opposition from within the party.