President Trump will address the nation tonight. If he takes legal action, it will likely be to declare a national emergency under the National Emergencies Act of 1976. The statute gives him wide leeway to decide what circumstances constitute an emergency. He will declare the emergency to take advantage of two additional statutes, 10 USC 2808 and 33 USC 2293, which would allow him to reallocate Department of Defense construction funds to build the wall.
Congress is the nation’s lawmaker, and is the one responsible for allocating funds to complete projects like the wall. These statutes allow the president to bypass Congress and reallocate funds that Congress has already appropriated, so long as those funds remain uncommitted to other projects. To use these statutes, the emergency the president declares must “require the use of armed forces,” so expect him to make that case in his speech as well.
For more details, see Professor Chesney’s Lawfare post.
I also spoke about this on KFBK earlier today and I’ll update this post with the link to that conversation when they have it posted on their website.