Acting White House chief of staff Mick Mulvaney strongly suggested on "Fox News Sunday" that President Trump will construct a border wall using his emergency powers if Congress cannot agree on a compromise to fund the construction in the next three weeks.

Trump on Friday signed a short-term spending bill to re-open the government through Feb. 15, ending the longest partial federal government shutdown in U.S. history. But the president insisted the move was not a "concession," writing in a tweet that "in 21 days, if no deal is done, it’s off to the races!" -- a message Mulvaney emphasized.

"We don't go into this trying to shut the government down. ... It’s still better to get it through legislation," Mulvaney, who officially still holds his title as Director of the Office of Management and Budget, said. "That's the right way to do it. At the end of the day, the president is going to secure the border, one way or another."

Mulvaney added, "At the end of the day, the president's commitment is to protect the nation, and he’ll do it either with or without Congress."

Meanwhile, speaking to NBC's "Meet the Press," Florida Republican Sen. Marco Rubio said that declaring a state of emergency would be "a terrible idea" and set "bad precedent."

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Mulvaney, speaking to fill-in anchor John Roberts, said that Democrats had privately indicated to the White House that striking a deal might be possible now that the government is temporarily re-opened.

"Why [Trump] did what he did was because many Democrats had come to us, some privately, many of them spoke out publically, but they actually started to agree with him on the necessity for a barrier on the southern border," Mulvaney told Roberts. "And they had come to us and said ‘Look, we agree with you, you’re winning the battle on the importance of a barrier on the southern border. But we simply cannot work with you while the government is closed.'"

"At the end of the day, the president’s going to secure the border, one way or another." — Acting chief of staff Mick Mulvaney

He added: "That’s a marked difference – excuse me, while the government was closed – that’s a marked difference from where the Democrat leadership was. They said they wouldn’t talk to us about border security ever. Nancy Pelosi famously said that even if we opened the government, she wouldn’t give us a single dollar for the wall. So I think the president saw a chance here to try and take the Democrats at their word."

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The White House has directed the Army Corps of Engineers to "look at possible ways of funding border security," including possibly using the president's emergency powers and unspent disaster relief money.

"I think the President wants his $5.7 billion," Mulvaney said. "Keep in mind – why why why is that number? It’s not a number that’s made up. It’s what the experts have told him. He’s listened to DHS. I’ve been in on the meetings. He’s listened to CBP [Customs and Border Patrol], he’s listened to ICE."

Should the White House move forward with an emergency declaration, it has a handful of legal routes to take. The National Emergencies Act grants the president broad authority to declare emergencies, and several federal laws then could clear a path for the White House to move ahead with building a wall.

One statute, 33 U.S. Code § 2293 - "Reprogramming during national emergencies," permits the president to "apply the resources of the Department of the Army’s civil works program, including funds, personnel, and equipment, to construct or assist in the construction, operation, maintenance, and repair of authorized civil works, military construction, and civil defense projects that are essential to the national defense."

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Another law, 10 U.S. Code § 2808 - "Construction authority in the event of a declaration of war or national emergency," permits the secretary of defense, in a presidentially declared emergency, to use "funds that have been appropriated for military construction" for the purpose of undertaking "military construction projects."

Mulvaney offered some advice to Democrats hoping to avoid the possibility of an emergency declaration: stop following their leader in Congress at every turn.

"The last time Democrats sort of followed Nancy Pelosi blindly down a path on policy, we ended up with Obamacare and bailouts and cap and trade and they lost control of the House," Mulvaney said. "The question is: Is Nancy really leading the Democrat Party or is she just being led by the hard left wing of her party. And will the rest of the Democrats follow her?

"The president from the very beginning here actually has been the one willing to negotiate," Mulvaney added. "He was the one staying in Washington, DC when Nancy went to Hawaii. He’s the one who stayed in DC when Democrats all went down to Puerto Rico. He was in D.C. when she tried to leave to go to Europe and the Middle East last week. He wants to negotiate on the deal. And the Democrat leadership simply refused to take him up on that."

Some Democrats have hinted they are willing to buck Pelosi. Last week, House Homeland Security Committee Chairman Bennie Thompson, D-Miss., broke with some of his fellow congressional Democrats by acknowledging in an interview that he "would not rule out a wall in certain instances," although he cautioned that the White House needs a better "plan" than simply using a wall as a "talking point."

Thompson suggested in an interview with ABC News' "This Week" that Trump was stuck with Pelosi, however.

"Mr. President, Democrats will work with you," Thompson said. "But you can't pick what Democrats you work with. We have picked our leaders, and you have to work with our leaders." He continued, "The notion that we can’t have barriers is just something that’s not true."

Democratic leaders have previously supported building border walls. Senate Minority Leader Chuck Schumer, D-N.Y., and other Democrats, including then-Sens. Barack Obama and Hillary Clinton, supported the Secure Fence Act of 2006, which authorized the construction of some 700 miles of fencing at the border. As of 2015, virtually all of that fencing had been completed, according to government figures.

And House Majority Leader Steny Hoyer, D-Md., acknowledged earlier this month on Fox News' "Special Report with Bret Baier" that border walls "obviously" work in some areas, and he rejected suggestions that barriers are necessarily improper -- contradicting Pelosi's claims that a wall is an "immorality."

Separately, Mulvaney said Trump has not taken the possibility of military force off the table in Venezuela. The government effectively collapsed in the socialist country this week, and top Republicans have warned that military action is possible -- especially if U.S. diplomatic officials there are harmed.

"I don’t think any president of any party who’s doing his or her job would be doing the job properly if they took anything off the table. So I think the president is looking at that extraordinarily closely."

Fox News has learned that the president and his entire foreign policy team is greatly encouraged by the fact that Venezuela’s Defense Attache in Washington has publicly turned against Nicolás Maduro and thrown support behind opposition leader Juan Guaido.

Fox News' Gillian Turner, Brooke Singman and Adam Shaw contributed to this report.