(Quick note: Dershowitz advanced a theory that it's not a crime for the President to try to get a foreign country to investigate a political rival, because it's perfectly natural for the President to think his reelection is what's best for America, and that he's therefore serving the national interest by violating the principles in the Constitution.)
Look: Dershowitz isn't stupid. He may play stupid on TV, but he's not stupid.
He is undoubtedly aware that the reason "treason" is defined *extremely* narrowly in the US Constitution is that kings will claim a person who acts against the desires of the king is committing an offense against the crown (i.e.: the state, not just the king).
So: Dershowitz is 100% aware that the founders wanted to create a stark separation between what's partisan/political, and what's nation-affecting, and they decided that crimes of "attacking the state" would be restricted to fighting the US, or giving aid and comfort to its enemies (and not merely it's "adversaries"). It would never, ever, be proper for the US to try to say someone (like a rival candidate) was working against the US, just because you don't like what that rival might do (like, "win the election").
If he were stupid and incompetent, he might not recognize how that plays out in the real world. He might really think something as pathetically stupid as "Trump thinks getting reelected is best for the nation, so it's okay for him to abuse the power of the state to secure that reelection". He's not that stupid.
And it's not like this is a difficult argument. First, there's the preamble to the Constitution. We, the people of the United States, ordained and established that Constitution. The President's service is to the people, not himself. And sure, it's perfectly human for a dolt like Trump to think he's the bestest ever because he's got so many media-fluffers saying he is. Nevertheless, no matter what the right wing media claims, trying to service himself, by attacking a private citizen, is flat out wrong, a violation of his oath of office, and of everything that our founders wanted for the United States.
Second, there's the Fourth Amendment; the government is supposed to keep out of the business of private citizens, unless there is cause to investigate them. The President violated that most sacred of duties, by deliberately trying to set up a fake investigation. Any other President might, just maybe, be able to argue "it's just an investigation, to answer some questions" but old WhinyPants McWitchHunt can't make that claim. He's been complaining about how horrible it is to be investigated, when there's plenty of cause for suspicion. He can't pretend he doesn't realize how serious his crime (requesting a baseless, bogus, investigation) really is.
Third, only the damnedest of fools would ever suggest that a person's personal feelings (like "it's better for the US if I'm reelected")should ever be permitted to override their sense of duty and responsibility. Face it, if a bank robber thinks stealing the bank's money, and putting it in circulation, is better for the local economy, that doesn't make it any less criminal, even though it's *not* an abuse of power granted under the Constitution to rob a bank. Why should personal feelings then excuse an abuse of power that is granted solely to serve the people?
Far down the list, we could note that Dershowitz made one claim in the past, and gave a completely self-serving reason for why he's correct now, claiming that *this* time, he's really thought about the matter. Since a lawyer is not forbidden from presenting a bogus legal argument, when defending a client, why should we pretend to accept that he's right this time, when he's paid to make a different argument? One would be wiser to assume the difference is the client's desires, not the considered legal opinion.
So, how crazy is he? Not very. Nor is he stupid. He is acting to protect the bad guys, he is advancing the most ludicrous of excuses, and he is trying to keep people in power who have shown that they will abuse their power, to the detriment of the United States.
There are a lot of words that might apply, but "crazy" is, alas, not one of them.