that letter is claiming that the president can order his associates to commit federal crimes and then can order federal law enforcement to ignore those crimes. It should be blindingly obvious to any sane person that this is not only wrong, but anyone arguing this should be removed from office.
In light of this we all know republicans will do nothing.
That's essentially what the defense is laying out, and the law that would put that in check is obstruction of justice. Well, Clinton was already impeached for that related to something vastly less in scope and totally unimportant to national security. That to me seems to completely undo their legal argument, at least to whether Congress should consider it an impeachable offense. But that's a political mechanism. This might be a defense in court if he were charged after leaving office and not pardoned. I think it's a stupid defense, but it helps elucidate to me the strategy that Trump's legal team had. They were making sure Mueller dotted his i's and crossed his t's, and established some degree of transparency such that they couldn't be blindsided, and they used that transparency to proactively submit documents and such to tie up Mueller's team, making it much harder to prove that getting more including Trump's testimony is required. And in attacking various sources, they are wanting to keep very tight reign on what quality of evidence is considered. Since there's a plethora of circumstantial evidence, any small victory in their arguments can be used to try and keep a bunch of crap out. It could work, because Trump is unpredictable and changes positions practically mid-sentence sometimes. If nothing else, there are good stall tactics, and having already crossed those bridges with the special counsel, they'll know how they might play out arguing them to Congress. As we know, he has plenty of shills that will use Trump's inconsistency to individually throw shade on a whole plethora of things. He can make it a purely political show very easily, if it wasn't already that anyway. Which sucks, because 2/3rds vote is required for removal. Perhaps the best hope is his own party turning on him, which doesn't seem too unlikely. Really, they would already have done that many times over if not for his popularity with their base. Really, they shouldn't care. It's not like those people are going to become Bernie Bros if Trump gets knocked out.