The problem with this defense on the merits is that the call wasn’t perfect. It was so clearly inappropriate that most of the professionals listening in real time were alarmed. The problem as a practical political matter is that maintaining the “perfect” line allows the president’s critics to score easy points every time another insider emerges to say he was disturbed by the call.
Meanwhile, Republicans have leaned heavily on the “no quid pro quo” argument that quickly emerged after the rough transcript of the call was released. The call doesn’t include an explicit quid pro quo, but it is suggestive of one, certainly combined with the unexplained withholding of defense aid to Ukraine. Here, too, more and more evidence has emerged — EU ambassador Gordon Sondland’s revised testimony is the latest example — that the aid package was conditioned on Ukraine’s committing to investigations that Trump wanted.
Overall, the White House and Republicans have been violating the first rule of a good defense counsel, which is not to deny things that are undeniable. It erodes your credibility and makes it harder to mount a better defense on other grounds.