WASHINGTON, D.C. — President Donald Trump capped months of speculation and two years of failed negotiations over fortifying the southern border by declaring an emergency on Friday, initiating almost certain legal challenges over the extent of his executive power as well as a new set of political risks.

“We’re going to confront the national security crisis on our southern border,” Trump said in an event at the White House Rose Garden in which he delivered a defense of his policies, touching on trade, drug control, the economy and the border.

“It’s all a big lie, a big con game,” he said of his opponents’ arguments against border barriers. “Walls work 100 percent,” he said.

Once he puts the emergency order into place, “we will then be sued … we will possibly get another bad ruling,” he said, reciting a litany of courts in a sing-song voice. “We’ll end up in the Supreme Court” where he hopes to “get a fair shake,” he said.

The declaration is intended to circumvent Congress, which has refused to spend the billions needed to deliver on Trump’s central campaign promise: a wall he had long insisted would be paid for by Mexico.

Administration officials say Trump will try to use emergency powers to divert money from other projects, mostly military construction efforts, to build or rebuild as many as 234 miles of border fences.

The emergency order would free an additional $6.6 billion for barrier construction, Mick Mulvaney, Trump’s acting chief of staff, told reporters.

That potentially would bring total spending on construction to $8 billion, including the $1.375 billion authorized by Congress in the spending bill that is set to reach Trump’s desk either Friday or today, when he will be at his vacation home in Florida.

The money would include $3.6 billion from an account for military construction projects and other funds from an account for projects to combat drug trafficking.

With the expanded military budget that he’s pursued, the Pentagon can afford to divert some funds to the border, Trump said. “This is a very, very small amount” of the military budget, he added.

The administration decided not to try the more politically controversial step of tapping disaster relief money intended to help Texas and Puerto Rico, Mulvaney said. Officials also abandoned plans that had been considered earlier to try to take money away from California water projects.

Officials declined to specify which specific projects would lose money or suffer delays as a result of the decision to shift funds, though they insisted the military’s readiness would not be diminished. They also declined to say where new barriers would be erected or rebuilt.

Building or repairing 234 miles was “our goal,” officials said, but made no promises.

“It’s going to be a little mix-and-match because instead of Congress just providing the money, the different pots have different authorizations for how and where we can use that money,” said a senior administration official who declined to be named under White House rules.

Trump has relaxed his demand for a solid structure that would cover the length of the border, yet still insists that hundreds of miles of steel bollard fencing are essential to the nation’s security.

So far, no additional miles of border fence have been built under his presidency, a contrast with each of his last three predecessors, but Trump has tried to persuade supporters that he’s making progress, in part by adopting a new slogan that implicitly takes credit for work previous presidents have done: “Finish the wall.”

Despite Republican control of both houses of Congress in his first two years in office, Trump was unable to strike a deal, in part because Democrats thwarted him, but also because fellow Republicans declined to make his wall their priority and he spurned deals that would have required him to accept compromises. Once Democrats took control of the House, his negotiating position worsened.

Congress could seek to overturn Trump’s emergency declaration, forcing him to use his veto power to defend his effort. As Trump predicted, opponents have already said they would challenge him in court.

In addition to a likely court fight over whether his emergency declaration is a valid use of presidential power, aggressive building plans could open Trump to confrontations with landowners along the border who have argued that building a barrier through their property violates their rights.

Fences already line nearly all the border from the Pacific Ocean through California, Arizona and New Mexico —just short of 700 miles, mostly through publicly owned land. But the more than 1,000 miles of borderland in Texas mostly lies in private hands and is mostly unfenced.

Trump, who is eager to show supporters he is keeping his promises, could opt to spend less money and build fewer miles if he believes his supporters will give him credit for the sheer act of boldness in declaring an emergency.

Even that support is far from a slam dunk, however.

Ann Coulter, the conservative commentator, called the declaration a “ruse” that “was always just a way to fool the rubes in his base” in a Thursday night tweet. She faulted Trump for accepting a bill passed by Congress on Thursday that funds just $1.375 billion for border barriers, but nothing for a wall, far less than the $5.7 billion Trump sought or the $25 billion over several years that he had rejected as part of a larger deal early last year.

Fox host Sean Hannity, another conservative media adviser, has been encouraging the national emergency move, writing on Thursday that “Trump keeps his promises. I predict he’ll find a way to get the money to build the wall.”

The decision epitomizes Trump’s tenure in the White House. While other presidents have wielded emergency authority, they have generally done so to sanction foreign adversaries or combat domestic crises, such as epidemics. Trump is pushing beyond what others have tried in pursuing an emergency declaration to fund projects that were explicitly rejected by Congress, which has the constitutional power of the purse.

Administration officials said Friday that presidents have used national emergency powers 58 times since 1976. Only two of those instances involved spending money, they said, pointing to orders signed by George H.W. Bush during the Iraq War in 1990 and by George W. Bush following the 2001 terrorist attacks. Officials said the authority was used in those instances to spend a combined $1.4 billion not authorized by Congress.