Impeachment 2.0: When Will The Backfire Begin?

Yesterday the House impeachment 2.0 team marched impeachment papers over to the Senate for trial. With even President Biden doubting a conviction of citizen Trump, is there a point where what increasingly appears to be a show trial backfires politically for the Democrats and for a president who is desperately trying to sell the slogan "unity" to a divided America? Is there any Constitutional provision for removing a president from office who has already been removed from office? Will this start a trend in retroactively impeaching past presidents? Or pro-actively impeaching presidential candidates?