Because of stuff he said

PowerLine: "It's official: On Trump's travel order, the Left is bonkers."  "Instead, they have to argue that it is “unconstitutional”–in the way in which liberals use that word, having no reference to the actual Constitution–only because it was issued by President Trump. They will eventually lose that argument. If it were accepted, it would effectively render the Constitution irrelevant. We would be living in a realm of pure, unconstrained politics."