On the home page, I write how I think the strongest ground for Republicans is what might be called the McGahn defense:

All of this bears some resemblance to Trump’s alleged obstruction of justice during the Mueller investigation. He hated the investigation and wanted it to go away, and even plotted against it, but at the end of the day, Robert Mueller did his work. More specifically, the Ukraine mess is lot like Trump’s order, or purported order, to then–White House counsel Don McGahn to fire Mueller. After drama, internal contention, and tragicomedy, nothing happened.