They have put themselves in checkmate.
Most people don't want this, even the ones that don't like him. Two and a half years of RussiaGate have turned the people against them on impeachment.
If they win impeachment, they lose face and they will lose the next election.
If they lose impeachment, it'll just be RussiaGate 2.
If they back out, they'll look weak.
Their anger has consumed their common sense and their foresight.