WASHINGTON – President Donald Trump threatened Friday to close the U.S. border with Mexico if Democrats keep opposing funding for his proposed wall, but he did not spell out how he would carry out such a policy or its broader impact.

"We will be forced to close the Southern Border entirely if the Obstructionist Democrats do not give us the money to finish the Wall & also change the ridiculous immigration laws that our Country is saddled with," Trump tweeted.

Trump has made the threat before but has not offered details. Illegal crossings are at the heart of his dispute with congressional Democrats over funding for a proposed wall along the U.S.-Mexico border.

It's also uncertain how closing the border would affect migration. Many people seeking to enter the United States claim political asylum, and the government is required to consider those requests regardless of border closures.

U.S. Customs and Border Protection reported that in November "51,856 people were apprehended between ports of entry on the Southwest Border."

There were an estimated 10.7 million undocumented immigrants already living in the United States in 2016, a 12-year low, according to the most recent statistics compiled by the Pew Research Center.

The partial government shutdown began after Trump refused to endorse a temporary spending bill because it lacked full funding for the border wall. Senate Democrats, meanwhile, blocked a revised plan that included wall money, describing Trump's barrier as an unnecessary waste of money that has now caused a temporary government shutdown.

"This isn't about security. It's Trump trying to get taxpayers to fund what Mexico won't – a monument to him at the border," said Democratic strategist Jesse Ferguson. "He's taken taxpayers hostage, and now he's upping the ransom demands."

Trump tweeted amid signs that the partial government shutdown that began a week ago will likely go on into the new year and the new Congress.

House and Senate leaders have said no votes are scheduled in the near term.

The new Congress is scheduled to convene Jan. 3, including a new Democratic majority in the House.

In an interview with USA TODAY this week, incoming House Speaker Nancy Pelosi said Democrats have no plans to support Trump's wall funding. She said the president is using fact-free "scare tactics" to stump for his wall, and has frequently changed his demands.

"First of all, the fact ... that he says, ‘we're going to build a wall with cement, and Mexico's going to pay for it’ while he's already backed off of the cement," Pelosi said. "Now he's down to, I think, a beaded curtain or something, I'm not sure where he is."

In a Friday morning tweet storm, Trump appeared to argue that closing the southern border would improve U.S. trade with Mexico, even though a full closure would presumably curtail the movement of goods and services over the border.

In November, U.S. officials closed the San Ysidro border crossing near San Diego, California, for five hours following a confrontation between migrants and U.S. authorities. The San Ysidro Chamber of Commerce later estimated a one-day loss of $5.3 million for more than 700 local businesses.

Sarah Pierce, a policy analyst with the Migration Policy Institute, said Trump could be talking about closing formal ports of entry like the one at San Ysidro. The government is already trying to harden the many areas between formal ports of entry,

"We're doing everything we can between points of entry," Pierce said.

In his tweets, Trump also again said he would consider cutting foreign aid to countries he claims are not doing enough to stop migration, specifically citing Honduras, Guatemala and El Salvador.

Reiterating his threat, Trump said in one post: "Either we build (finish) the Wall or we close the Border."

In October, just a few weeks before pivotal congressional elections, Trump threatened to close the southern border.

"I must, in the strongest of terms, ask Mexico to stop this onslaught - and if unable to do so I will call up the U.S. Military and CLOSE OUR SOUTHERN BORDER!" he wrote on Twitter.

Mexican officials have said they are trying to work with U.S. officials to keep asylum seekers in Mexico until their claims move through U.S. courts.

Trump also threatened to close the border – "permanently if need be" – in a tweet on Nov. 26, weeks after elections in which Democrats won control of the U.S. House and as the wall funding dispute was building.