With Trump, though, the only clear precedent being set is one of deplorable incompetence. He’s taking unpopular action that divides his party and unites the opposition, he’s doing so with a combination of brazen hypocrisy and nonsense rhetoric that makes the power grab impossible to cloak, he’s guaranteeing himself an extended legal battle — and he isn’t even accomplishing any obvious goal (there’s a reason real immigration restrictionists are against this plan) except the personal one of saving a tiny bit of face.
This spectacle will not prevent some future president from abusing an emergency declaration more effectively. But the idea that Trump’s grab enables future abuses more than the moves that Bush and Obama made is extremely dubious. If anything, precisely because his contempt for constitutional limits is so naked and his incompetence so stark, Trump has (modestly, modestly) weakened the imperial presidency by generating somewhat more pushback than his predecessors.
So the emergency declaration is not itself a constitutional emergency. Rather, as often in the Trump presidency, it’s a moment that illuminates how a more dangerous would-be autocrat might someday act.