The lawfare site has a list of seven theories of L'Affair Russe, ranging from the most innocuous, to the most treasonous.Actually, I believe the opposite; if Trump somehow manages to stagger all the way to the 2020 finish line without being removed, voters will gladly get rid of him.
But if he's forcibly removed before the 2020 election? There's gonna be an enormous group of aggrieved voters saying that their votes had been overturned. Trump, the newly-minted martyr, will be right back in front of all his rallies, pushing for himself, or (if he can't legally run again) some Trump proxy to get into office. And that'll make for a very interesting election indeed...
Theory of the Case #1: It’s All a Giant Set of Coincidences and Disconnected Events
Theory of the Case #2: Trump Attracted Russophiles
Theory of the Case #3: The Russian Operation Wasn’t Really About Trump at All
Theory of the Case #4: Russian Intelligence Actively Penetrated the Trump Campaign—But Trump Didn’t Know
Theory of the Case #5: Russian Intelligence Actively Penetrated the Trump Campaign—And Trump Knew or Should Have Known
Theory of the Case #6: Kompromat
Theory of the Case #7: The President of the United States is a Russian Agent
While it would please me if Trump was impeached for incompetence (because that's what he is), the slam dunk cases for removal are six and seven. Anything less serious risks blowback.
Three, four and possibly five indicate a serious threat to national security that need to be addressed in a "non-partisan" manner, but it's more of a stretch to impeach over that. You could imply that five is a serious, potentially disqualifying lapse in judgement, but if the democrats want Trump gone in less than four years it's a bit of a gamble.
I'd like to have Republicans thinking that they were quite foolish to elect Trump. I'd rather they didn't think of him as a martyr.
There will be other rationales for impeachment, but Russia,is at present, the sexiest.