But they can't impeach Trump now... They have to wait for the House to finish the investigation, which could take a month or 2, then the house has to vote, then the senate has to schedule the proceedings and have their trial. We may be looking at early 2020 before they could actually complete the impeachment process.
And remember, the GOP seems to be tying themselves to Trump, through things like cancelling primaries in some states, and linking his personal campaign to the GOP. All that will need to be undone in the event of an impeachment.
And of course, Trump is unlikely to go quietly if he is voted out... I'm sure he will be giving us hourly updates on how the GOP were all "never Trumpers" to begin with, which will keep his base riled up.