Carrots and sticks. Right now, no one is offering any carrots. So GOP members are stuck, just as you say, weighing the threats.
But here's a heretical thought: Assuming you normally wouldn't, would you be willing to vote for a Republican who voted for impeachment? Would you be willing to give political cover in this one instance? If not, then you aren't part of any Republican's calculations. And if you wouldn't vote for them under any circumstance, you aren't part of there considerations. Ever.
Seriously, as long as people are so polarized (on both sides) that they won't consider voting for someone on the other side no matter what, there is no incentive for politicians to listen to the other side.
Hold out a carrot, if you can stomach it, and things might change.