Office of Management and Budget (OMB) Director and incoming Acting White House Chief of Staff Mick Mulvaney told "Fox News Sunday" that the ongoing partial government shutdown will likely extend into the new year, as President Trump insists on funding for a border wall and "refuses to go along to get along."

Mulvaney also asserted that House Minority Leader Nancy Pelosi, who is weeks away from reclaiming her role as speaker of the House, is intentionally stalling negotiation efforts to resolve the partial closure, which began on midnight Saturday and affects a quarter of the federal workforce.

"It's very possible this shutdown will go beyond the 28th and into the new Congress," Mulvaney said. He added that federal workers will receive pay through that date, and that only future pay periods will be affected.

"I think there's an implication here for Nancy Pelosi's election for the speakership," Mulvaney continued. "I think she's now in that unfortunate position of being beholden to her left wing, to where she cannot be seen as agreeing with the president on anything until after she's speaker."

The Senate is set to reconvene on Dec. 27, and sources on both sides of the aisle have told Fox News that Washington could be in for a "very long" shutdown that lasts deep into January.

A senior source told Fox News on Sunday that the White House has taken over negotiations from Alabama GOP Sen. Richard Shelby, who chairs the Senate Appropriations Committee. Shelby had been working with Democrats and reportedly making progress, contributing to a sense of optimism on Capitol Hill on Friday.

Mulvaney pointed to the image of the so-called "steel slat barrier" fencing that Trump tweeted on Friday as a potential source of compromise during negotiations. The president called such a barrier "totally effective while at the same time beautiful."

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"That's what we want to build," Mulvaney said, referring to the picture in Trump's tweet. "In the Democrats' mind, that is not a wall. So they have offered us $1.3 billion to build the barrier that we want -- but then they go on TV and say there's no money for the wall. We've already told the Democrats we want to build what the president tweeted out. It doesn't have to be a 30-foot tall concrete [wall]."

Early Sunday, Trump tweeted that while "drones and all of the rest are wonderful and lots of fun," only a "good old fashioned wall" is truly effective.

Mulvaney said Democrats previously had offered $1.6 billion for the barrier just two weeks ago, and by now offering $1.3 billion, they were "moving in the wrong direction" -- perhaps, he suggested, because of Pelosi's political ambitions.

A Democratic source on Sunday told Fox News the latest offer from the White House was $2.1 billion for border security including new fencing, plus $400 million, which the source described as “slush fund for immigration priorities.” If those numbers are accurate, the proposal would represent a compromise from the White House's original $5 billion border wall request to approximately $2.5 billion -- but both parties remain concerned about the optics of "blinking" first in the showdown.

Speaking on CBS' "Face the Nation," Delaware Democratic Sen. Chris Coons said that while there is "frankly no path toward his getting $5 billion in American taxpayer money to meet his campaign promise of a 'big, beautiful wall' with Mexico" -- nevertheless, "there is a path toward responsibly appropriating about $1.3 billion for border security."

In an interview later in the day with ABC's "This Week," Mulvaney -- who will soon replace outgoing White House Chief of Staff John Kelly -- said he and Trump had joked about a recorded clip of Mulvaney in 2016 saying Trump is a "horrible human being."

"We joked about it last night," Mulvaney said. "What's wrong with Washington, D.C.? People spend a lot more time looking at what people say instead of what they do."

When asked whether Trump was right that Mexico would ultimately pay for the border wall, Mulvaney responded, "Technically, you and I both know that it cannot work exactly like that."

But the president, earlier this month, wrote on Twitter that "MEXICO IS PAYING FOR THE WALL" through the favorable terms in the new trilateral trade agreement between Mexico, Canada, and the U.S. Although Congress has yet to approve the U.S.-Mexico-Canada-Agreement (USMCA), which will replace the North American Free Trade Agreement (NAFTA), it contains various new measures championed by Republicans that are intended to strengthen free trade in North America.

White House Press Secretary Sarah Sanders announced over the weekend that Trump had canceled his planned Christmas vacation to his Mar-a-Lago estate amid the shutdown, and that first lady Melania Trump -- who had already made the trip to Florida -- will return to Washington to spend Christmas with her husband.

"This is what Washington looks like when you have a president who refuses to go along to get along," Mulvaney told host Chris Wallace.

The main sticking point for border wall negotiations was funding for Trump's signature 2016 campaign promise of a wall on the southern border. Trump had demanded $5.7 billion for wall funding, and a bill with that funding attached passed the House on Friday. But efforts have derailed in the Senate, where 60 votes were required for passage under current filibuster rules, and therefore Democrat votes are needed in conjunction with support by the GOP.

HERE'S WHAT TRUMP'S $5 BILLION IN BORDER WALL FUNDING WOULD BUY

Democrats have poured cold water on the idea that they would support anything close to that. Senate Minority Leader Chuck Schumer, D-N.Y., said Saturday that the wall was a "bone to the hard right" and that they had proposed $1.3 billion for "border security."

"I’ve heard the president and his allies in the media say that Democrats don’t support border security. Nothing could be further from the truth. Democrats have always been for smart and effective ways to secure our border," Schumer said. "We are pushing for technology, like drones and sensors, and inspection equipment."

The Senate adjourned Saturday afternoon and was not due to meet for a scheduled session until Thursday. Early Saturday evening, the House also concluded for the day.

Vice President Pence and Mulvaney arrived at the Capitol Saturday afternoon to meet with Schumer to continue negotiations for an end to the stalemate. Senate Majority Leader Mitch McConnell, R-Ky., said that "productive discussions are continuing."

"When those negotiations produce a solution that is acceptable to all parties -- which means 60 votes in the Senate, a majority in the House, and a presidential signature -- at that point, we will take it up here on the Senate floor," he said.

Separately on Sunday, Mulvaney defended Trump's planned troop withdrawal from Syria, saying that while "we recognize the fact that this unpopular within the beltway" and "we recognize this fact is unpopular within the Defense Department," the move is ultimately "very popular with ordinary American people."

Mulvaney additionally commented that outgoing Defense Secretary Jim Mattis was not the right man for the job.

"Let’s be honest with each other," Mulvaney said. "I think the relationship between these two men [Trump and Mattis] had been fraying. I think the president no longer relied on Mattis to be able to deliver the president’s vision. The president has to know – I don’t have to agree with the president on everything he asks me to do. In fact, we’re not going to. I think he hires people who disagree with him because he likes that different input, ok? But if I’m physically incapable, completely incapable, of doing what he wants me to do just because I don’t believe it, I cannot serve him well."

But Leon Panetta, who previously served as CIA Director, Secretary of Defense, White House Chief of Staff and OMB Director, later told Wallace on "Fox News Sunday" that the administration's move to withdraw from Syria -- apparently over Mattis' repeated and strenuous objections -- was irresponsible.

"I think the step to suddenly withdraw those troops from Syria has sent a terrible message to the world about where the United States stands," Panetta said.

He added: "If we’re sending a message to ISIS that somehow they can come back, they can be resilient, they can, at some point come back and attack us and have us vulnerable to another 9/11 attack -- that’s unacceptable."

Fox News' Chad Pergram and Mike Emanuel contributed to this report.