Photo: The Canadian Press

U.S. Border Patrol agents stand guard on the other side of the border in San Diego as they prepare for the arrival of hundreds of pro-migration protestors, seen through the border fence from Tijuana, Mexico, Monday, Dec. 10, 2018.

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.

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.