Rachel Maddow was discussing this yesterday. Almost all of the Trump situations / transgressions / potential criminal involvements that Muller investigated occurred in 2017. The statute of limitations on these events lapses in five years 2022.
If impeachment is not pursued now, and if, for some extraordinarily crooked reason, Trump should retain the presidency, he will run out the calendar on the investigated incidents (which comprise the basis for impeachment), and he would, effective 2022, become a free man. (Not factoring in the New York State investigations.)
Using the frustrating Mueller guideline that one cannot indict a sitting president, should Trump lose the 2020 election and become a private citizen in January, 2021, he can still be prosecuted for whats already on the books.
Think Trump wants to stay in office much? He will fight like a cornered animal, because losing could mean a trip to prison.
Start impeachment proceedings immediately. Chasten that bastard NOW. No conviction possible? Screw it, thats not the goal, which is to keep him on the defensive, put him under stress and pressure from multiple directions to exhaust that monster, and have his approval ratings tank lower than ever. If his people and Russia conspire again to throw the election, America will be screwed beyond recognition...forever.
Impeach now. Its not a sure thing, but its the right thing to do. And I seriously doubt that itd hand the election over to Trump. That notion presupposes that there are scads of Trump voters out there who are considering voting for a Democratic candidate. Thats such a bullshit line as if those who worship their white-nationalist Neanderthal figurehead could be lured away from their fuhrer.