Alan Dershowitz isn't anything close to a constitutional law conservative or originalist. He's an unabashed supporter of the "living Constitution" idea, supported judicial activism in terms of gay marriage even while acknowledging the Constitution required no such thing, etc..
Dershowitz has defended Trump because support for Israel is a hugely important issue for him, and Trump is just much, much better than any of the Democrats on Israel. So really, he's just an ally of convenience right now. Which is still better than having an enemy.