1. Losing Control Of the Process
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2. Oh Hi, Hunter Biden
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2. (sic) Senators On The (Campaign) Trail
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4. Republican Unity
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5. The Odds Of Winning Are Not Good
Losing control of the process is a biggie. Schiffty, et al, could not block Rs' witnesses or stifle lines of questioning.
I think the next biggie, though, is that it would be public. The public would see that Dems' accusations are groundless. The public would see the credibility of Dems' witnesses impeached. The public would see that Trump's actions were normal and reasonable.
Building on the latter, it would come out that the doings of Hunter Biden were odoriferous, and Joe's actions to protect him were acts of corruption.
Further, the whole process of how the Dems came to impeach President Trump would be scrutinized. It will look really ugly under that scrutiny.
Election 2020 is coming. Many Dem voters will be disgusted by the lunatic actions of their party. OTOH, those same lunatic actions will galvanize R voters: the Dems could lose control of the House, lose more seats in the Senate, and reelect Donald Trump.