Some things come to mind here:
They are trying to goad the GOP into impeachment because they can paint Biden as a victim and repeat the Clinton impeachment, which in their mind, was a victory.
They want Biden gone for Newsom.
They need cover for the obvious two-tiered justice system. They want Trump in jail with the appearance of a “fair” justice system.
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I don’t think they want Republicans to discover Impeachment.
It’s a gentleman’s agreement that only Dems can use it.
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