Who are these people whose only reason for voting for Trump in 2020 will be impeachment?

I keep hearing the argument that trying to impeach Trump will hurt the dems in 2020, but how is this calculation being made? Clinton got a boost in the polls after the Republicans went after him mostly because people didn't think an affair was grounds for impeachment... do you think that people are going to be as sympathetic towards Trump for getting help from the Russians, trying to fire investigators, directing people to lie and the host of other crimes that will be dug up?

The fact of the matter is, if all of Trump's dirt coming to the light gains him more support, then we're fucked anyway. Might as well impeach him.