This is all one needs to know about is action standing up to judicial scrutiny:
"In Mr. Trump’s case, he is defining a longstanding situation at the border as an emergency even though illegal crossings have actually fallen in recent years. And unlike either of the Bushes, he is taking action after failing to persuade lawmakers to go along through the regular process."
I can think of a couple very strong, foundational arguments the Left can come up with that Trump’s declaration of an National Emergency was not justified. Those in favor of the wall might not want to celebrate too hard at this point.