President Donald Trump is again considering invoking emergency powers to build his proposed wall on the U.S.-Mexico border without congressional approval, roiling the latest bipartisan negotiations over immigration with the renewed threat of unilateral executive action and further dividing Republicans already reeling from the fallout of the shutdown.

"The president's commitment is to defend the nation, and he will do it either with or without Congress," acting White House Chief of Staff Mick Mulvaney said on "Fox News Sunday."

The partial shutdown, which was the longest in U.S. history, ended Friday with Trump agreeing to temporarily reopen the government without any money for a wall.

Inside the West Wing over the weekend, Trump told advisers that declaring a national emergency may be his best option now as he scrambles to assert himself in a divided government and to secure wall funding, according to four people involved in the discussions who were not authorized to speak publicly.

One White House official described Trump's decision to reopen the government as "clearing the deck" for executive action rather than a retreat. And a longtime confidant said Trump has grown increasingly frustrated by news coverage of his concession to Democrats and has been encouraged by conservative allies to escalate the fight.

A bipartisan, bicameral congressional committee has been charged with brokering an agreement on border security as part of a deal to keep the government open past Feb. 15, and a stalemate could trigger another shutdown.

Trump's consideration of such action comes as conservative commentators have lashed out at Trump and said he gave in to top Democrats on Friday, and as special counsel Robert Mueller's probe into Russian interference in the 2016 election has moved deeper into Trump's inner circle, raising questions about the future of his presidency.

Some Republicans on Sunday waved Trump off potentially signing a declaration after 35 days of enduring criticism as he held firm. Public opinion polls show that the public blames Trump and Republicans more than Democrats for the shutdown.

Sen. Marco Rubio, R-Fla., called the prospect of a national-emergency declaration a "terrible idea," reflecting widespread conservative unease about using executive powers in sweeping ways to achieve political ends, a tactic they have long criticized Democratic presidents of employing.

"It's just not a good precedent to set in terms of action. It doesn't mean that I don't want border security. I do. I just think that's the wrong way to achieve it," Rubio said on NBC's "Meet the Press."

Sen. Roy Blunt, R-Mo., agreed. "I happen to agree with the president on barriers at the border and border security as an important first step, but there might be a future president that I don't agree with that thinks something else is an emergency," Blunt said on "Fox News Sunday," adding that he hopes "the president doesn't have to go there."

Other Republicans said the GOP appetite for using emergency powers was stronger than the remarks of lawmakers on television suggested, because of widespread thought that the party's base would applaud Trump for being bold.

"He's certainly going to have Democratic opposition for partisan reasons and Republicans opposed based on the precedent it sets," former White House legislative director Marc Short said in an interview. "But there is one thing some Republicans say to the media, and then there is what they say quietly to each other when the camera is not on: 'I sure wish he'd do it.' "

Several White House officials said privately on Sunday that Trump has argued that a national-emergency declaration in the coming weeks could pressure Congress to include wall funding as part of a broader legislative package next month and could signal to the GOP's core voters that the president is going to extremes to secure funding for his campaign's biggest pledge.

Mulvaney said that if the legislation Congress sends to the president's desk is unsatisfactory, Trump could veto it. He said on CBS' "Face the Nation" that Trump may be prepared to bring about a shutdown next month.

After Trump agreed to reopen the government, a committee was charged with negotiating an agreement on border security as part of a new spending bill for the Department of Homeland Security.

Republican leaders appointed to the committee include Sen. Richard Shelby of Alabama, the chairman of the Senate Appropriations Committee, along with GOP Sens. Blunt, Shelley Moore Capito of West Virginia and John Hoeven of North Dakota. Democratic leaders tapped Sens. Patrick Leahy of Vermont, Richard Durbin of Illinois and Jon Tester of Montana.

The White House Counsel's Office, led by Pat Cipollone, has prepared drafts of declarations, and Trump spent much of Thursday night reviewing them in the White House residence as he watched TV coverage of the shutdown, according to two White House officials familiar with the discussions who were not authorized to speak publicly.

Angry with Democrats' refusal to bend to his demands, in particular with House Speaker Nancy Pelosi, D-Calif., Trump pressed Cipollone for guidance about the potential legal repercussions and called friends, such as Fox Business host Lou Dobbs, to hear their views about the negotiations, the two officials said.

In the end, Trump backed off on Friday after being briefed by aides on mounting anxiety among Republican lawmakers over the shutdown and federal flight delays. Trump was wary of the prospect of a shutdown showdown and court challenges over emergency powers unfolding at the same time, the officials added.

"Ultimately he'll be judged by what happens at the end of this process, not by what happened this week," Mulvaney said on Fox.

The White House declined to comment about the internal deliberations or the president's calls.

There are tensions in the White House about the political cost of using emergency powers. Senior adviser Jared Kushner has reservations and is hopeful that Democrats may eventually agree to work with the administration on a bipartisan immigration deal, while others, including White House trade adviser Peter Navarro, have talked up a national emergency as a way of prompting a reckoning on the issue, according to the four people familiar with the discussions.

Some Trump allies have urged the president to act, stoking his own instincts about using emergency powers, White House officials said.

"Come February the 15th, if the Democrats still say: 'Go to hell on the wall, you get a dollar, that's it,' they basically tell Trump, 'I'm not going to do with you what I did with Bush and Obama,' then I hope he will go the emergency route," Sen. Lindsey Graham, R-S.C., told Fox News on Friday.

The Trump administration has spent weeks casting the situation at the U.S.-Mexico border as a security and humanitarian crisis that may necessitate the declaration of a national emergency, laying the groundwork in congressional briefings, news conferences and Trump's address to the nation this month.

Trump argued on Sunday that illegal immigration was costing the country tens of billions of dollars a month, although it was not clear on what data he was basing his estimate.

"We are not even into February and the cost of illegal immigration so far this year is $18,959,495,168," he tweeted. "Cost Friday was $603,331,392."

Trump has previously claimed that the cost of illegal immigration is more than $200 billion a year, without providing any evidence for those claims.

About 11 million people are estimated to be living in the United States without documentation. But on Sunday, Trump challenged that number, tweeting that "there are at least 25,772,342 illegal aliens, not the 11,000,000 that have been reported for years, in our Country. So ridiculous! DHS"

Asked on "Face the Nation" about that number, Mulvaney said he did not know where Trump was getting his information. But he argued that the figure "has to be larger than 11 million" because of the numbers of migrants who continue to cross into the United States each month.

"I think that number was accurate a couple of years ago. We know that it's going up," Mulvaney said.

As the negotiations begin anew, lawmakers from both parties stuck to their positions on Sunday. Some Republicans, however, cautioned against another government shutdown over the wall.

Sen. Susan Collins, R-Maine, said on "Face the Nation" that "absolutely nothing" was accomplished by partially shutting down the government.

"Shutdowns are never good policy, ever," she said. "They are never to be used as a means to achieve any kind of goal, no matter how important that goal may seem to be."

The Washington Post's Dan Lamothe contributed to this report.