To win over the die-hards, it would have to be clear and convincing. Maybe two or three witnesses testifying about firsthand accounts of Trump making secret arrangements with Putin ahead of the election, or something like that. I'm assuming that virtually none of this is in writing anywhere, so finding documents will be hard (just a guess, unless the Russians just started being stupid all of a sudden).

I think without the die-hards being convinced, their Congresspeople and senators won't be so inclined to impeach.