President Trump declared a national emergency to fund his border wall. Here are some of the challenges that could crop up to block his declaration. (Video: Jenny Starrs/The Washington Post, Photo: Jabin Botsford/The Washington Post)

This post has been updated.

President Trump’s threat to declare an emergency because Congress won’t give him $5.7 billion for a wall, designed to respond to talk-radio hosts' hysteria, will mark a new phase in his presidency. Rather than provide an “off-ramp" so he can declare victory in an unwinnable standoff, its most likely result will be the erosion of his power as a result of intensified judicial scrutiny and shattering of his support in the Republican Party.

Let’s begin with the obvious: There is no illegal immigration emergency (border crossings have diminished dramatically in recent years); to the extent there is a humanitarian crisis of asylum-seekers, it is not one a wall would address. The president has made it plain: The emergency didn’t exist when Republicans controlled both houses of Congress. It will not exist if Congress capitulates, allocating billions that will be spent only after years of litigation. That’s certainly not an “emergency,” as the term is used in everyday parlance (“a serious, unexpected and often dangerous situation requiring immediate action”). Every president who has declared an “emergency” has used it in the context of an actual emergency — 9/11, a hurricane, a forest fire, etc.

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Former White House counsel Bob Bauer explains that “a president has the discretion to declare an emergency, not to create one.” He continues:

This distinction is critical. Congress did not attempt to define what constitutes an “emergency,” but this is not evidence that it left the choice without limits to the president. Rather, as one commentator wrote, the absence of a definition reflected “the assumption … that genuine crisis is readily identifiable by everyone in the polity.” Hence the law’s reference to “the period of a national emergency,” which would be clear to all, and which would then—and only then—be subject to the president’s declaration at his or her discretion.

The federal district and circuit courts have not been shy about rebuffing Trump’s executive overreach (on the Muslim travel ban, sanctuary cities, child separation, limits on asylum, environmental actions and more). They are highly unlikely to accept the president’s argument that an emergency is whatever he says it is and that it can be used to defy Congress when it refuses to exercise its power of the purse in ways he doesn’t like. (There is a process for those fights: Congress passes a bill, the president vetoes it, and Congress may override it.)

Will the Supreme Court, if it gets that far, bail him out? The Supreme Court, as Chief Justice John G. Roberts Jr. made clear when he rebuffed Trump’s suggestion that there were “Obama judges” and “Trump judges,” understands all too well this president’s design on extensive powers and his assertion that his powers are absolute, unrestrained by the Congress or the courts. Before we get to any issue involving Russia or the special counsel, the Supreme Court will find the authority of the judicial system challenged. While Trump’s appointees may indulge him, Roberts has demonstrated an understanding of the court’s institutional credibility. Getting the Supreme Court, even one as lax on executive powers as this one, to roll over would be a heavy lift.

If Trump heads for the emergency exit and reopens the government, he very likely will be rebuffed while the government resumes operation. By the time this works its way through the courts we will be at the end of the fiscal year, where presumably this very same fight will be replayed. His promise for Mexico to pay for the wall has already gone up in smoke; when he has no wall to show for all his histrionics, he will be hard-pressed to declare victory. The courts in all likelihood will have confirmed that Trump (and his successors' powers) are curtailed by the Constitution and the courts. His ambition and his extravagant claims of executive authority will be limited. The winner will be the courts and Congress.

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Aside from losing and arousing the courts' determination to curtail an imperial executive, a declaration of emergency power will open fissures in the GOP. We’ve already seen the Wall Street Journal editorial page and the National Review editors (certainly not members of the #NeverTrump camp) warn against the potential power grab. We’ve see Republican Sens. Charles E. Grassley, Susan Collins, Lisa Murkowski and Marco Rubio all express concerns about using an emergency declaration in this context (although it’s far from clear what they do about it if Trump proceeded against their advice). Republican governors of states such as Texas, Florida and Arizona — from which Trump would snatch emergency monies for the wall — would be hard-pressed to go along with Trump’s charade. Trump is always most emphatic about matters on which he is on the thinnest ice (No contacts with Russia! Never said Mexico would pay!), so his insistence that Republicans are united should be an additional “tell” that they are not.

Trump has painted himself into a corner, but his proposed escape hatch provides no relief. He’s a diminished president whose inclination to double down only accelerates his decline. It’s hard to imagine at this point that he wouldn’t face a primary challenge from within his party. After all, what do Republicans have to lose by dumping him?

UPDATE: Sen. Rob Portman (R-Ohio) joins other Republican members of the Senate warning against an emergency declaration. “It my hope is that the president doesn’t go the national-emergency route because of the precedent it sets,” he told me. "It’s a much wiser idea to negotiate something in Congress and reach an agreement here.”

Read more:

Trump has conjured a crisis out of thin air. That should worry us all.

I worked in the Justice Department. I hope its lawyers won’t give Trump an alibi.

Sherrod Brown as Trump-slayer? Here’s his big idea.

Jennifer Rubin: All the reasons there is no ‘emergency’

The Post’s View: Congress gave the president too many powers. Now it must scale them back.

The Post’s View: Here are some real emergencies. None of them requires the president to turn into a dictator.