Part of me thinks that is why Pelosi is dead set on holding off impeachment. And why she's super fucking pissed off at the Cunt Squad.
They know they are fucked both in terms of taking back the Senate and White House in 2020. BUT if they can just hold the House, they can finally pull the trigger on impeachment to distract the masses from their colossal loss. And use the investigative powers of the House to create such a shit storm that they can bully the bare minimum of Republicans in the Senate to get Trump impeached.
Bonus points, they can go full tilt petty cunt mode and file impeachment papers probably within hours of Trump being announced as winning re-election and completely ruin what would be his moment of triumph and have the media focus on impeachment over Trump getting re-elected.
But that hinges on them keeping the House. And if they lose the House, they won't have another chance to regain it until 2022 and by then Trump's Presidency will be just about over and the masses definitely won't have any tolerance for the Democrats pulling that shit.
They are "holding off on impeachment" because they have jack fucking shit. We'd be so lucky as for impeachment to be allowed to proceed, because it would be the most hilarious "um um um HE SAID THINGS THAT HURT OUR FEE-FEES" spectacle you've ever seen. He's committed no crimes and the only actionable offense he did was selling weapons to Saudi Arabia, something Chocolate Jesus also did so investigating him for that would have him bringing up tons of shit the "scandal-free" Obama did and didn't get hauled to impeachment for.
But lets pretend that Donald Trump did conspire with Putin to deny Hilary Clinton HER TURN to sell America to corporate and foriegn interests for personal gain. Lets say they are right, Trump is personally torturing children in cages with cattle prods. Right now their evidence is fucking weak, laughingly week, like tinfoil conspiracy weak. If they try to impeach him now, and fail, and then suddenly Putin comes out with a book "If I Did It" where he talks about how he would have used Russian Hackers to make Meme-magic real, and it turns out its not theory, that's what actually happened, Congress won't get a second bite at the apple.
They also don't want to impeach trump because doing that will let Trump call defense witnesses. So depending on what they charge him with, he could do some really amazing shit. He can call Obama before congress to make him explain how much he knew about FISA abuse on record - no executive privilege, as he's not a sitting president anymore. He can make Hilary and her entire staff (the ones who don't commit suicide right before testifying, at least) why they paid a foreign agent for piss-porn fan fiction, and how federal money was funneled to pay for it through her staffer and his wife.
Trump WANTS them to impeach him.
The other thing to consider is that even if they impeach him, being impeached doesn't require being removed from office. You can be impeached and basically just censured, with as I understand it means you aren't allowed to go on the official Congressional Record anymore without prior approval from Congress, and there is a note in the offical congressional record from that year about you being a very, very bad person