The original problem is that a border wall was Trump's signature promise, backed by his guarantee-"Believe me!"-that Mexico would pay for it. Trump has successfully induced his supporters to shrug off the abandonment of his promise that "Mexico will pay for the wall," which is an impressive hustle already. He might well have induced them to forget the wall, or to accept that a light upgrade of the existing 700 miles of fencing counted as "the wall." By now, his supporters are much more invested in the idea of Trump as a success than in his achieving any success in particular.
Still, Trump has to imagine that in 2020 some political opponent will drive to an unfenced part of the U.S.-Mexico border with a camera crew, walk back and forth across it, and make a mocking campaign commercial asking: Whatever happened to the wall?
Trump's bizarre, rambling announcement of a national emergency
He felt the need to be seen doing something, at least. The state of emergency is that something. But this something comes with four catches.
The first catch is legal. The declaration of a state of emergency is heading almost immediately to court. Construction could be enjoined while the litigation proceeds. Trump could lose. Yes, that would give him somebody to blame in 2020. Liberal judges stopped the wall. But a loss with an excuse remains a loss.
By declaring an emergency, the president gains legal authority to move around some military-construction funds, reportedly about $3.6 billion. But that money has to come from somewhere, and where it comes from is other projects. "That must have been really tough. To lose. To be a loser." Those were Trump's mocking words to Acting FBI Director Andrew McCabe, as quoted in his new book. He'll have to wear them himself if the courts stop his wall.
The legal route imposes another risk. Few voters will understand the limits on the emergency powers Trump has just invoked. The invocation will sound to many like final confirmation that Trump aspires to dictatorship. If the courts stop him, he will look like a defeated dictator-dangerous but weakened.