President Donald Trump on Sunday threatened to shut down the federal government this fall unless Democrats give in to his border security demands, including by giving him billions more for his proposed southern border wall.

Should Democrats continue denying Trump his border barrier and other demands and the president make good on his high-stakes threat, it would be the third funding lapse of his tenure. It also would shutter the government just weeks before voters will decide which party controls the House and Senate — and the Trump-GOP agenda — come January.

“I would be willing to “shut down” government if the Democrats do not give us the votes for Border Security, which includes the Wall!” he wrote.

[Trump Takes Full Ownership of Economy As Midterms Approach]

Trump called for lawmakers to send him a bill that ends the visa lottery system and the policy known as “catch-and-release,” meaning detaining undocumented migrants and releasing them into the country while they await a court hearing. He also seemed to make his long-stated call for an “Immigration [system] based on MERIT!” a requirement to keep the government operating this fall.

If he keeps the latter on his list of demands, because Democrats oppose such a system and have the votes in the Senate to block legislation, a government shutdown would be inevitable.

The president, who constantly plays to his conservative base, ended his tweet with the kind of nationalistic immigration rhetoric that roils Democratic lawmakers: “We need great people coming into our Country!”

I would be willing to “shut down” government if the Democrats do not give us the votes for Border Security, which includes the Wall! Must get rid of Lottery, Catch & Release etc. and finally go to system of Immigration based on MERIT! We need great people coming into our Country!

— Donald J. Trump (@realDonaldTrump) July 29, 2018

On his proposed U.S.-Mexico border wall, Trump got $1.6 billion last year. A House spending bill for fiscal 2019 proposes $5 billion for the project, while a Senate measure proposes the same amount as 2018.

It was not immediately clear if Trump was referring to an extra $5 billion in fiscal 2019 White House officials floated in recent months or full funding for the project, which the administration has estimated to cost $18 billion to $25 billion.

[11 Eyebrow-Raising Comments From Trump’s Midwest Swing]

But Senate Appropriations Chairman Richard C. Shelby, R-Ala., and other spending-focused lawmakers have told Trump personally several times that the $1.6 billion amount likely is all that can get through the Senate this year.

That means Trump could be headed for a showdown not only with Democrats, but also senior members of his own party.

“We told the president that this is a big down payment,” he told reporters on June 28. “We’re trying to work with him. We’re trying to work with our colleagues, and I think border security is very important in this country.

“The president would like more money,” Shelby said, “but all presidents do.”

Trump’s threat comes just four days after he had a meeting at the White House with Speaker Paul D. Ryan and Senate Majority Leader Mitch McConnell to discuss the government funding strategy.

The GOP congressional leaders urged Trump to suppport their strategy of prioritizing the enactment of smaller packages of appropriations bills before the Oct. 1 start of the fiscal year and using a continuing resolution to punt the deadline on the remaining bills until after the election.

Ryan walked away with the impression they were all on the same page, which included deferring the debate on the Homeland Security appropriations bill that includes wall funding until after the midterms.

“It’s not a question of if, it’s a question of when,” the speaker told reporters Thursday. “The president is willing to be patient.”

Watch: Odds of a Government Shutdown Tick Up as House Leaves for Recess With Unfinished Business

Kellie Mejdrich contributed to this report.