Mark Tushnet, a highly respected professor of Constitutional law at Harvard (formerly at Georgetown), has an op-ed in the Washington Post explaining what is likely to happen if Trump decides to declare a national emergency and divert billions of dollars from other places in the federal government to pay for the wall he wants.
The Constitution says the president can’t spend money that hasn’t been allocated for a specific purpose by Congress, but a statute called the National Emergencies Act allows him to divert money from the military construction budget and a few other places in case of a national emergency. He can also divert money set aside for disaster relief. In total, it’s about $15 billion he could hypothetically access if he declares and emergency. The first thing that could happen is Congress could rescind that authority:
The courts could also get involved, and almost inevitably would because there would be lawsuits filed. Those suits would likely focus on when the president can invoke an emergency and under what circumstances:
The House and the Senate can use a fast-track procedure to pass a joint resolution effectively taking away the president’s power to use those funds — leaving him without any appropriation to pay for the wall.
Joint resolutions are statutes, though, which means that Trump can veto this one — and then Congress gets a chance to override the veto, by a two-thirds vote in both chambers. Reports suggest that Senate Majority Leader Mitch McConnell (R-Ky.) has been advising the president that there’s a decent chance that he’d lose such a veto fight.
Enter the courts. The National Emergencies Act doesn’t define what “emergency” means. The term has to have some limits, though. How can something be an emergency in late February when you saw it coming two months — or years — earlier? (Maybe an asteroid crashing into the Earth — law professors make our living thinking about hypotheticals like that.)
More important, saying that something’s an emergency simply because the president hasn’t been able to persuade Congress to appropriate money seems to stretch the word pretty far — and to give the president a powerful tool to do an end run around a Congress he regards as obstructing what he thinks, but it doesn’t, are national priorities.
But the courts typically show enormous deference to the president on matters of national security. Regardless of the outcome, the matter would be working through the courts into 2020 and the next election and he probably would be enjoined from doing it until the Supreme Court rules on it. So the next possibility is that he gets voted out. Or perhaps booted out due to impeachment or stepping down as a result of the Mueller and other investigations. So there are many ways this could go if he decides to invoke that authority.