Republican and Democratic appropriators from both chambers plan to meet Monday afternoon in an effort to revive spending talks as the government heads toward its second shutdown in three months.

The so-called “four corners” — Senate Appropriations Chairman Richard C. Shelby, R-Ala., and ranking member Patrick J. Leahy, D-Vt., and House Appropriations Chairwoman Nita M. Lowey, D- N.Y. and ranking member Kay Granger, R-Texas, — will attend the meeting, according to a Shelby spokeswoman.

A Democratic aide said the meeting would take place at 3:30 p.m.

Negotiators disclosed Sunday that their talks had hit a snag, with Shelby blaming the problem on Democratic insistence to limit the number of beds available for Immigration and Customs Enforcement arrests inside U.S. borders.

“The talks are stalled right now,” the Alabama Republican said on “Fox News Sunday.” Also on Sunday, acting White House Chief of Staff Mick Mulvaney said on “Meet the Press” “you absolutely cannot rule out” a shutdown.”

Meanwhile, President Donald Trump is headed to El Paso for a Monday rally where he will laud the benefits of constructing a wall to protect the southern border. Trump previewed what could be his message in a Monday morning tweet: “The Democrats do not want us to detain, or send back, criminal aliens! This is a brand new demand. Crazy!”

Current funding for a portion of the government runs out at midnight Feb. 15 unless House and Senate conferees can resolve the impasse over Trump’s demand for border wall funding and Democratic demands regarding ICE beds.

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, while 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.

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.

Law enforcement groups have been lobbying against the Democrats’ interior enforcement cap, which was first outlined in a proposal from House Democrats released Jan. 31. “In order to meet the cap being tentatively proposed by Congress, ICE would be compelled to release thousands of aliens from custody,” representatives of the National Sheriffs’ Association and Major County Sheriffs of America wrote to top appropriators in a letter dated Feb. 8.

Shelby tweeted his support for the sheriffs’ position Monday morning. “Our law enforcement professionals warn against restricting ICE’s ability to detain dangerous criminals. We should listen to them,” he wrote.

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.

“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.

Kellie Mejdrich and Jennifer Shutt contributed to this report.