Migrants, part of a caravan of thousands traveling from Central America en route to the United States, run across the Tijuana River to reach the border wall between the U.S. and Mexico in Tijuana, Mexico, on Sunday. (Alkis Konstantinidis/Reuters)

President Trump has signaled to congressional Republicans that he will not budge from his demand of $5 billion for the construction of a wall along the Mexico border, heightening chances for a partial government shutdown at the end of next week.

Senate Appropriations Committee Chairman Richard C. Shelby (R-Ala.) told reporters Monday that the president has made clear that the $5 billion is a red line and he won’t take less. Democrats have rejected that figure and have agreed to $1.6 billion.

It’s unclear how the difference can be bridged, especially with clashes along the U.S.-Mexico border spotlighting what Trump says is the need for greater security and physical barriers. Although the bulk of the federal government has been funded through next fall, money for the Department of Homeland Security and a handful of other agencies runs out at midnight Dec. 7 unless Congress and the president act first.

“The president, he’s interested in $5 billion; how do we get there? You can configure it in different ways,” Shelby told reporters as the Senate returned to session after a week-long Thanksgiving break.

Asked if the $5 billion figure was a red line for the president, Shelby responded: “That’s what he tells us.”

Shelby added that the president has not shown a willingness to settle for less. “Not from my conversations with him,” the senator said. A White House spokesman had no immediate comment.

Shelby’s comments suggested a hardening standoff as a partial shutdown looms and Republicans prepare to lose their majority in the House, and with it their best chance of getting Trump the money he wants for his unfulfilled campaign promise to build a wall along the U.S.-Mexico border. Trump has long promised Mexico would pay for the wall, even as it has become clear that U.S. taxpayers will foot the bill.

Trump has made ultimatums before only to later back down, but some people close to the president believe he is more dug in this time, believing it may be his last chance to secure sizable money before Democrats take control of the House of Representatives in January.

One option under consideration would spread the $5 billion over two years, according to a source familiar with the discussions, who spoke on the condition of anonymity to describe confidential deliberations.

Trump himself has raised the prospect of a shutdown repeatedly and suggested that it could be a good time for one in light of the migrant caravan approaching the border. On Sunday, U.S. authorities fired tear gas into Mexico as Central American migrants attempted to scale border fencing — a show of force Trump defended Monday, saying authorities “had to” do it.

On Thanksgiving, Trump told reporters that a shutdown over the wall “could happen.”

“Could there be a shutdown? There certainly could,” Trump said. “And it will be about border security, of which the wall is a part.”

House Republican leaders will meet with Trump at the White House on Tuesday to discuss the issue. Earlier this year, GOP leaders persuaded Trump to postpone a fight over his border wall until after the election, concerned such a conflict could hurt them in the midterms.

Now Trump appears to want that fight — even as congressional GOP leaders try to talk him out of a shutdown.

“I don’t think it’s a good idea to shut the government down, period,” said Shelby, who met with Trump before Thanksgiving along with Majority Leader Mitch McConnell (R-Ky). “But people are going to posture, and it could happen; it’s happened before.”

Senate Minority Leader Charles E. Schumer (D-N.Y.) said earlier this month that lawmakers should stick with the bipartisan $1.6 billion deal agreed to earlier this year in the Senate.

“We believe Democrats and Republicans should stick with their agreement and not let President Trump interfere,” Schumer said. “Every time he interferes, it gets bollixed up.”

Congressional aides said negotiations were ongoing, but it’s unclear how much progress they’re producing. If agreement can’t be reached, Congress might end up trying to pass a short-term spending bill that extends current funding levels, but it’s uncertain whether Trump would go along with that.

Democrats are hoping to use the year-end spending bill to advance priorities of their own. Schumer said Monday that he wants to attach legislation to protect special counsel Robert S. Mueller III to the spending bill. But Shelby dismissed that idea, saying, “I’ve never heard of anything like that being included.”

Seung Min Kim contributed to this report.