On a national level, it's still far from clear that a majority is opposed to impeachment or that it's making people like Trump better - see my reply to Greg. And as for particular districts, I remember discussing not too long ago, although I can't recall if you were included in that conversation, that even the polls quoted by Trump's reelection campaign to support the theory that it's killing Democratic House candidates in vulnerable districts, showed that the whole thing was making very little difference either way. It might still happen - but I doubt it.

Of course, Trump could still win even without such a bump, and I don't think there's any scenario in which the Dems gain five Senate seats without also winning the presidency. The four seats needed if they do win the presidency are already unlikely.

So I don't think Pelosi has much to lose with impeachment. And yeah, what she has to gain is making the last Republican moderates, Collins especially, choose between the devil and the deep blue sea. Take a stand and get torn apart by rabid Trump fanboys in a Republican primary, or swallow their principles and with it their credibility towards moderate voters in the general. Romney and Murkowski aren't facing reelection this year and have reasons to believe they could survive either way, but Collins...

I suppose the best tactic for McConnell if he plays the long game is to let Romney and/or Murkowski vote to impeach, with Collins voting against but making clear she's nevertheless absolutely not on board with what Trump did.