U.S. President Donald Trump sought Tuesday to pressure Democratic congressional leaders into supporting his demand for billions of dollars to build his promised wall along the U.S.-Mexico border, threatening to have the military build it “if Democrats do not give us the votes to secure our Country.”

Trump tweeted the threat hours before Senate Democratic Leader Chuck Schumer and House Democratic Leader Nancy Pelosi were to meet with Trump at the White House in an effort to avert a possible partial government shutdown on Dec. 21, when funding for some agencies is scheduled to expire.

In a series of tweets Tuesday, Trump said immigration and border patrol agents and thousands of active-duty service members he sent to the border have done a “FANTASTIC” job. But he said “A Great Wall would be, however, a far easier & less expensive solution.”

Trump said he looked forward to meeting with Schumer and Pelosi, but claimed they don’t want border security for “strictly political reasons.”

“If the Democrats do not give us the votes to secure our Country, the Military will build the remaining sections of the Wall. They know how important it is!” Trump said.

Walter Shaub, former head of the government ethics office, tweeted a reply, "In their free time? Because they can’t use DOD’s appropriations for that. They are not the White House’s personal staff of coat checkers and gardeners. They don’t work on the leader’s hobby horses. This isn’t the Saudi kingdom."

Schumer and Pelosi said Monday that Republicans have the power to keep the government open since they control Congress and the White House.

“Our country cannot afford a Trump Shutdown,” they said in a statement, adding that Trump “knows full well that his wall proposal does not have the votes to pass the House and Senate and should not be an obstacle to a bipartisan agreement.”

Republican congressional leaders have repeatedly said it’s up to Trump to cut a deal with Democrats, an acknowledgment of their inability to produce spending bills with Republican votes alone.

That gives Democrats some momentum heading into the closed-door talks, which also could veer into Trump’s request for emergency funding for deadly wildfires in California and a Republican-sponsored bill to extend expiring tax breaks and delay some health care taxes.

Before lawmakers adjourn for the year they also may consider a bipartisan criminal justice reform bill, a bill to protect special counsel Robert Mueller and a plan to overhaul the system for handling sexual harassment complaints on Capitol Hill.

By far, the biggest unresolved issue is the border wall. Trump wants the next funding package to include at least $5 billion for it, an idea Democrats have flatly rejected.

Pelosi and Schumer have urged Trump to support a bill that includes a half-dozen government funding bills largely agreed upon by lawmakers, along with a separate measure that funds the Department of Homeland Security at current levels through Sept. 30. The homeland bill includes about $1.3 billion for fencing and other security measures at the border.

If Trump rejects that, Democrats will likely urge a continuing resolution that funds all the remaining appropriations bills at current levels through Sept. 30, an aide said. The aide was not authorized to discuss strategy by name and requested anonymity.

Trump said Friday that Congress should provide all the money he wants for the wall and called illegal immigration a “threat to the well-being of every American community.” At an appearance in Kansas City, Missouri, Trump accused Democrats of playing a political game and said he ultimately would win.

“I actually think the politics of what they’re doing is very bad for them,” Trump said of Democrats. “We’re going to very soon find out. Maybe I’m not right. But usually I’m right.”

Pelosi, who is seeking to become House speaker when the new Congress convenes in January, said she and many other Democrats consider the wall “immoral, ineffective and expensive” and noted that Trump promised during the 2016 campaign that Mexico would pay for the wall, an idea Mexico has repeatedly rejected.

Protecting borders “is a responsibility we honor, but we do so by honoring our values as well,” Pelosi said last week.

Schumer said Democrats want to work with Trump to avert a shutdown, but said money for border security should not include the concrete wall Trump has envisioned. Instead, the money should be used for fencing and technology that experts say is appropriate, Schumer said.

“We do not want to let a Trump temper tantrum govern our policies or cause the shutdown of a government, which everyone on both sides of the aisle knows is the wrong idea,” Schumer said. If Trump “wants to shut down the government over Christmas over the wall, that’s his decision,” he said.

Vermont Sen. Patrick Leahy, the top Democrat on the Senate Appropriations Committee, said Trump is the only obstacle between fully funding the government and a shutdown.

“Time and again, President Trump has used the government of the American people as a bargaining chip for his fabricated solution to his manufactured crisis,” Leahy said Monday in a Senate speech.

Trump “wants to score a made-for-reality-TV moment and he doesn’t care how many hardworking Americans will suffer for it,” Leahy said. “This is not about border security. This is about politics, pure and simple.”

House Majority Whip Steve Scalise, R-La., said Democrats were the ones playing politics.

If there’s a better way to secure the border than the $5 billion plan Trump has laid out, Democrats “need to come with an alternative,” Scalise said Monday on Fox News Channel. “They can’t come and say they want to shut the government down for no reason because they don’t want border security. They’ll lose that argument with the American people.”