Negotiations on a border security deal have hit a snag in a dispute over immigrant detention policy, Senate Appropriations Chairman Richard C. Shelby said Sunday.

House and Senate conferees were scrambling to reach a deal by Monday that would resolve the impasse over President Donald Trump’s demand for a border wall and avoid another partial government shutdown when current funding runs dry on Feb. 15. But Shelby put the odds of a deal at only “50-50,” citing a partisan rift over Immigration and Customs Enforcement, or ICE.

“The talks are stalled right now,” the Alabama Republican said on “Fox News Sunday.” “We’ve got some problems with the Democrats dealing with ICE that is detaining criminals that come into the U.S. They want a cap on them. We don’t want a cap on that.”

Acting White House Chief of Staff Mick Mulvaney told NBC’s “Meet the Press” that “you absolutely cannot rule out” a shutdown.

 Watch: As lawmakers begin to hash out border security, how do conference committees work?

Trump has sought enough bed space to house an average daily population of 52,000 migrant detainees; House Democrats want to cut that to 35,520 for the rest of the fiscal year ending Sept. 30, phasing out family detention completely by then. A bipartisan Senate version of the Homeland Security bill, approved last June on a 26-5 vote, would have provided funding for 40,520 detention beds.

The latest dispute centers on a Democratic demand to cap the number of detention beds available for undocumented immigrants arrested when they are already inside the U.S., as opposed to those arrested when trying to cross the border.

Democrats are seeking a cap of 16,500 beds for those arrested inside the country. Republicans have refused to make a counteroffer to the Democrats’ most recent border security proposal until that demand is dropped, according to a Democratic aide. “So we are not talking right now,” the aide said.

The Democratic aide said 16,500 is the level of such arrests from the last three months of President Barack Obama’s administration. By contrast, the comparable level for the Trump administration as of Feb. 4 was 20,700, according to the aide, about 25 percent more than in late 2016.

“A cap on ICE detention beds will force the Trump administration to prioritize deportation for criminals and people who pose real security threats, not law-abiding immigrants who are contributing to our country,” House Homeland Security Appropriations Subcommittee Chairwoman Lucille Roybal-Allard, D-Calif., said in a statement Sunday.

Republicans have said they would agree to such a cap only if it excludes criminals from the count, a senior GOP aide said. But that condition would make the cap “totally useless,” the Democratic aide said.

In a Sunday afternoon tweet, Trump gave his assessment of the current state of play.

“The Border Committee Democrats are behaving, all of a sudden, irrationally,” he wrote. “Not only are they unwilling to give dollars for the obviously needed Wall (they overrode recommendations of Border Patrol experts), but they don’t even want to take muderers (sic) into custody! What’s going on?”

The Border Committee Democrats are behaving, all of a sudden, irrationally. Not only are they unwilling to give dollars for the obviously needed Wall (they overrode recommendations of Border Patrol experts), but they don’t even want to take muderers into custody! What’s going on?

— Donald J. Trump (@realDonaldTrump) February 10, 2019

The 16,500 cap on detainees arrested inside U.S. borders was initially included in the House Democrats’ Homeland Security spending proposal outlined on Jan. 31.

Money barrier

Lawmakers were still wrestling with how much money to provide for physical barriers along the border, as sources said the final figure could hover somewhere around $2 billion. That would be less than half of the $5.7 billion Trump has sought this fiscal year for his border wall.

Mulvaney, appearing also on Fox, declined to say whether Trump would sign a bill that includes about $2 billion for border barriers. He said the White House message to negotiators was, “We’ll take as much money as you can give us,” but the administration would seek to supplement that amount on its own.

Mulvaney said the administration could tap other funds through reprogramming and still has the option of redirecting additional money by declaring a national emergency. If all such options were used, he said, “The whole pot is well north of $5.7 billion.”

Lawmakers of both parties have counseled Trump against declaring an emergency to fund the wall, saying it would set a troubling precedent that could be abused by future presidents. Mulvaney said the emergency option remains on the table but Trump “would prefer legislation because it’s the right way to go.”

Democrats have been reluctant to provide any more than the $1.6 billion for border barriers that was part of a bipartisan Senate Homeland Security bill last year. But in recent days, they have expressed a willingness to consider more money for “enhanced fencing” if it were part of a package that included more funding for personnel and technology improvements on the border.

“Negotiations seldom go smooth all the way through,” Sen. Jon Tester of Montana, the ranking Democrat on the Senate Homeland Security Appropriations Subcommittee, said on Fox. “I’m very hopeful, not positive, but very hopeful we can come to an agreement.”

But Mulvaney said he wasn’t sure how much money Democrats were willing to bring to the table for border security. “I don’t think they know where they stand on this particular issue, and I think that’s one of the reasons why we’re having a difficulty coming to a deal,” he said.

A deal on border security would clear the way for passage of seven unfinished spending bills for the fiscal year that began last October. The bills, which amount to about $320 billion in discretionary spending, are the Homeland Security, Agriculture, Financial Services, Transportation-HUD, State-Foreign Operations, Commerce-Justice-Science, and Interior-Environment measures.

Shelby said Monday remains the deadline for a deal by the House-Senate conference committee that is negotiating a Homeland Security bill. “I’m not confident we’re going to get there,” he said. “I’m hoping we’ll get there. The next 24 hours are crucial.”