President Donald Trump renewed his threat to invoke a national emergency as a way to circumvent Congress and build a wall on the southern U.S. border, depending on what happens in ongoing talks to end a partial government shutdown now into its third week.

“I may declare a national emergency dependent on what’s going to happen over the next few days,” Trump told reporters Sunday before departing to the Camp David presidential retreat in Maryland for what he termed “meetings on Border Security and many other topics” with senior White House staff.

Democrats have already threatened a legal challenge if Trump tries to declare a national emergency. “He’ll face a challenge, I’m sure,” Senator Dick Durbin, Democrat of Illinois, said Sunday on CBS’s “Face the Nation.”

Republican Senator Richard Shelby of Alabama said on Fox News that Trump has some “inherent powers” to bypass Congress, but that border funding must be done the right way, via legislation.

A delegation of top White House aides led by Vice President Mike Pence will meet again with congressional staff on Sunday after a two-and-a-half hour discussion Saturday was unable to forge a deal. Trump said on Twitter that “not much headway” was made on Saturday, and told reporters on Sunday that the next meeting is unlikely to produce results either.

“I think you’re going to have some very serious talks Monday, Tuesday, Wednesday,” Trump said. “We have to have border security.”

Trump said on Friday he could declare a national emergency to circumvent lawmakers and build the wall, though budget experts said Congress would still need to allocate the funds. Asked about it on “Fox News Sunday,” White House Press Secretary Sarah Huckabee Sanders said “whatever action he takes will certainly be lawful, and we’re looking at every option we can.”

Washington Democrat Adam Smith, the incoming chairman of the House Armed Services Committee, said Trump has the authority to declare an emergency and have the U.S. military build the wall but that such an action would likely be challenged and would be a bad use of defense spending.

“In this case, I think the president would be wide open to a court challenge saying, ‘Where is the emergency?’” Smith said on ABC’s “This Week” on Sunday. “You have to establish that in order to do this.”

Trump and Democrats are at an impasse over the president’s demands for $5.6 billion to build a wall, which has caused a partial government shutdown that entered its 16th day on Sunday. While Democrats have offered funding for border security, House Speaker Nancy Pelosi has called a wall “immoral.”

Trump is willing to rule out concrete for a wall if that helps reach a deal with Democrats, acting White House Chief of Staff Mick Mulvaney said.

“If he has to give up a concrete wall, replace it with a steel fence in order to do that so that Democrats can say, ‘See? He’s not building a wall anymore,’ that should help us move in the right direction,” Mulvaney said on NBC’s “Meet the Press” in one of two appearances on Sunday morning political shows.

Democrats have insisted that Trump support reopening the government while the negotiations on the border wall continue and asked on Saturday for a formal budget justification for the administration’s position to determine what the White House’s request on a border wall is, said a person familiar with the discussion.

House Democratic Leader Steny Hoyer said Sunday on NBC’s “Meet the Press” that the suggestion of a steel fence versus a concrete wall “will be discussed,” that “we need to open up the government first” before talking about it.

Trump said last month that he would “take the mantle of shutting down” the government over border security, and he believes the standoff could be solved in 10 minutes, Mulvaney said in an appearance on CNN’s “State of the Union” on Sunday.

But the White House and congressional Democratic negotiators can’t even agree on “basic facts,” and Democrats are stalling because they think Trump is losing the battle over public opinion, he said.

“The Democrats, for better or worse, think they are winning this battle politically,” Mulvaney said.

Still, Trump has said contradictory things in recent weeks about what would constitute a wall. The president tweeted on Dec. 18 that “we are not building a Concrete Wall, we are building artistically designed steel slats, so that you can easily see through it.”

After former Chief of Staff John Kelly suggested in an interview with the Los Angeles Times published Dec. 30 that the idea of a concrete wall had been nixed long ago and “to be honest, it’s not a wall,” Trump tweeted the next day that “an all concrete Wall was NEVER ABANDONED, as has been reported by the media.”

Sunday’s negotiating session will follow a two-and-a-half hour meeting Saturday between Pence, senior Trump adviser Jared Kushner, Homeland Security Secretary Kirstjen Nielsen and staff for House and Senate leadership.

Meanwhile, the shutdown is having wide-reaching effects. Airlines can’t get permission to add new planes to their fleets, mortgage lenders can’t verify the income of borrowers and the Food and Drug Administration can’t accept many new drug applications.

Trump was tweeting about the wall early on Sunday, quoting past comments in support of a barrier by former president Barack Obama, whose name he misspelled, and Senator Hillary Clinton and insisting the wall will end illegal border crossings and and crime.

“The only reason they do not want to build a Wall is that Walls Work!” Trump tweeted.