Anyone voting to impeach him should be primaried. You don’t know that his opponent will lose in the general election.
The fact that someone might lose a Republican seat should not be the criteria of whether you support them. That is how we have Mitch McConnell and Lindsey Graham. We don’t dare vote them out because a Democrat might win the seat. In most cases, the Republican is only nominally a Republican because they know that no matter what they do as long as they are Republican, we will help them keep the seat.