"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!" Trump tweeted.
Trump's Twitter threat introduced a fresh wild card for congressional negotiators to deal with as the remaining legislative days tick down before government funding runs out -- an explosive prospect for both parties with just 100 days until the midterm elections.
Trump has previously floated the possibility of a government shutdown over border security and immigration, and on Sunday he made his threat explicit, saying he would do so unless Congress funds his proposed wall, which he promised Mexico would pay for, and puts in place his preferred immigration policies.
In May, Trump suggested "closing up the country for a while" if he did not get his wall.
"They don't want the wall," Trump said. "But we're going to get the wall, even if we have to think about closing up the country for a while."
Sunday's shutdown threat from Trump also echoed a remark he made in February when he said "I'd love to see a shutdown" if the government did not agree to address immigration.
Congress ultimately passed a spending bill in March that funded the government through September. Trump threatened at the time to veto the spending agreement, but eventually signed the bill while expressing his displeasure with Congress.
"I said to Congress, I will never sign another bill like this again," Trump said in March.
A shutdown over Trump's wall at the September deadline would mark the third lapse in appropriations this year, following a shutdown in January as Democrats battled with the Trump administration and congressional Republicans on protections for "Dreamers" as well as a brief shutdown when Republican Sen. Rand Paul of Kentucky blocked a spending vote.
Senate Majority Leader Mitch McConnell announced in June that he was canceling much of the Senate's August recess, saying the chamber needed the additional time to make progress on Trump's nominees and pass appropriations bills. And with the House out on August recess, there's not much time left before the deadline that both chambers will be in session.
Both Speaker of the House Paul Ryan and McConnell met with Trump last week to discuss funding the government.
Asked about Trump's Twitter threat, a pair of Republican lawmakers said Sunday that they didn't think the government should or would shut down this September.
"Let's hope not," Wisconsin Sen. Ron Johnson said on CBS's "Face the Nation."
Johnson added that he didn't think a shutdown would be "helpful" to Republicans in the November elections, "so let's try and avoid it."
Ohio Rep. Steve Stivers, the chairman of the National Republican Congressional Committee, said on ABC's "This Week" said he did not think it would come to a shutdown.
"I don't think we're going to shut down the government," Stivers said. "You know, I think we're going to make sure we keep the government open, but we're going to get better policies on immigration."