WASHINGTON - We have a national emergency, all right. Its name is Donald Trump, and it is a force of mindless, pointless disruption.

The president's decision to officially declare an emergency -- to pretend to build an unbuildable border wall -- is not only an act of constitutional vandalism. It is also an act of cowardice, a way to avoid the wrath of Ann Coulter, Rush Limbaugh and the rest of the far-right commentariat.

It is an end-run around Congress and, as such, constitutes a violation of his oath to "preserve, protect and defend the Constitution of the United States" -- which gives Congress, not the president, the authority to decide how public money is spent. It does not give Trump the right to fund projects that Congress will not approve. Authoritarian leaders do that sort of thing. The puffed-up wannabe strongman now living in the White House is giving it a try.

Let's be clear: There is no emergency. Arrests for illegally crossing the U.S.-Mexico border peaked in 2000, nearly two decades ago, at more than 1.5 million a year. They declined sharply under Presidents George W. Bush and Barack Obama and, in 2017, were at their lowest point since 1971. In 2018, apprehensions ticked up slightly -- but still barely climbed above 400,000.

There has indeed been an increase in families presenting themselves at legal points of entry to seek asylum -- those groups of bedraggled Central Americans that Trump calls "caravans." Under U.S. and international law, these people have an undisputed right to ask for asylum and have their cases evaluated. Again, they come to legal border crossings to seek admission. Only a literal handful try to navigate the forbidding rural terrain where Trump says he wants to build a wall.

What the administration really needs to do is expand and improve facilities for processing, caring for and, when necessary, housing these asylum seekers. But Trump doesn't care about doing the right thing, or even the necessary thing. He cares only about being able to claim he is following through on his vicious anti-immigration rhetoric, which brands Mexican would-be migrants as "rapists" and Central Americans as members of the MS-13 street gang.

Trump had two years in which Republicans controlled both the House and the Senate -- and could not convince Congress to give him funding for a wall. He decided to make it an issue only after Democrats won the power to say no. The president's negotiating strategy -- pitching tantrums, walking away from the table, venting on Twitter, provoking the longest partial government shutdown in history -- was never going to work. You might think he'd have learned something about how Washington works by now, but you would be wrong.

Since there obviously is no legitimate emergency, Trump's declaration -- and the shifting of resources from duly authorized projects to the wall -- will surely be challenged in court. It is possible, if not likely, that any actual construction will be held up indefinitely.

Indeed, legal briefs arguing against Trump's action practically write themselves. An emergency, by definition, is urgent. The 9/11 terrorist attacks, for example, were clearly qualified as a national emergency. But Trump has been talking about issuing an emergency declaration to build the wall for a couple of months. If such action wasn't necessary in December, some judge will surely ask, then why now?

Money for the wall will have to be taken from other projects, all of which have constituencies in Congress and among the public. Ranchers and others whose land would have to be taken by eminent domain for the wall will be up in arms.

Politically, Trump carelessly put Republican senators in a tough spot. Speaker Nancy Pelosi, D-Calif., may have the House pass a resolution of disapproval, which the Senate would be compelled to take up. Senate Majority Leader Mitch McConnell, R-Ky., and his caucus would have to decide whether to support a presidential power grab they know is unwise -- or oppose Trump and risk the ire of the GOP base.

One of the most strident Republican criticisms of Obama was that he took executive actions that should have been the purview of Congress. But this action by Trump goes much further and sets a dangerous precedent.

What would keep the next Democratic president from declaring an emergency, in the wake of some mass shooting, and imposing a ban on assault weapons? Is that what McConnell wants as his legacy?

Trump cares only that his base is mollified. And that nobody remembers how Mexico was supposed to foot the bill.

Eugene Robinson's email address is eugenerobinson@washpost.com.