Mick Mulvaney, all-around dogsbody for the Russian asset in the Oval Office, went on Sean Hannity's show Wednesday and repeated Trump's threat to use executive authority to declare an emergency at the border to get his wall.
"[I]f Congress won't participate or won't go along, we'll figure out a way to do it with executive authority," he told Hannity. "With, and let's be clear about this, legal executive authority. […] This is not a circumstance of the President overstepping his bounds." Congress would indeed have something to say about that.
As Mark Sumner has explained, Congress—specifically House Speaker Nancy Pelosi—can immediately force a vote on rolling it back, "and Trump's attorneys have made it clear to him that an attempt to sidestep the constitutional roles of Congress and the executive will result in a swift trip to court."
Not even Mulvaney, the guy who will clean up any of Trump's messes in any of the roles Trump wants him to take on, can rewrite the Constitution for him.