It would have to be such a massive shift in support of impeachment among republicans that the chance of it happening is unlikely.
The way I see it, republicans have a choice:
- Stick behind Trump. Probably lose the next election, but at least put in a decent fight
- Impeach Trump and get slaughtered in 2020. (Yes, many republicans will probably stuck with the party, but in elections, a few percentage points of support can make a big difference, and there are probably enough hard-core Trump fans that the republicans would be in real trouble if they turned on him.)