– President Donald Trump, who repeatedly insisted during the 2016 campaign that Mexico would pay for a wall along the southern border, is privately pushing the U.S. military to fund construction of his signature project.

Trump has told advisers that he was spurned in a large spending bill last week when lawmakers appropriated only $1.6 billion for the border wall. He has suggested to Defense Secretary Jim Mattis and congressional leaders that the Pentagon could fund the sprawling project, citing a “national security” risk.

After floating the notion to several advisers last week, Trump told House Speaker Paul Ryan, R-Wis., that the military should pay for the wall, according to three people familiar with the meeting last Wednesday in the White House residence. Ryan offered little reaction to the idea, these people said, but senior Capitol Hill officials later said it was an unlikely prospect.

Trump’s pursuit of defense dollars to finance the U.S.-Mexico border wall underscores his determination to fulfill a campaign promise and build the barrier despite resistance in the Republican-led Congress. The administration’s last-minute negotiations with lawmakers to secure billions more for the wall failed, and Trump grudgingly signed the spending bill Friday after a short-lived veto threat.

Four days later, Trump continued to express regret over signing the $1.3 trillion package, which funded the government and averted a shutdown, saying it was a mistake and he should have followed his instincts.

In another interaction with senior aides last week, Trump noted that the Defense Department was getting so much money as part of the spending bill that the Pentagon could surely afford the border wall, said two White House officials who spoke on condition of anonymity. The Pentagon received about $700 billion in the spending package, which Trump repeatedly lauded as “historic.”

Meanwhile, the $1.6 billion in the bill for some fencing and levees on the border not only fell far short of the $25 billion that Trump was seeking, but it came with tight restrictions on how the money could be spent.

White House press secretary Sarah Huckabee Sanders deflected a question Tuesday about money dedicated to the military being used to fund construction of the wall. “I can’t get into the specifics of that at this point, but I can tell you that the continuation of building the wall is ongoing, and we’re going to continue moving forward in that process,” Sanders told reporters.

The president has suggested to Mattis that his department, instead of the Department of Homeland Security, could fund the construction, two Trump advisers said. But the military is not likely to fund the wall, according to White House and Defense Department officials.

The Pentagon has plenty of money, but reprogramming it for a wall would require votes in Congress that the president does not seem to have. Taking money from the current 2018 budget for the wall would require an act of Congress, said a senior Pentagon official.

“First Mexico was supposed to pay for it, then U.S. taxpayers, and now our men and women in uniform? This would be a blatant misuse of military funds and tied up in court for years. Secretary Mattis ought not bother and instead use the money to help our troops, rather than advance the president’s political fantasies,” said Senate Minority Leader Chuck Schumer, D-N.Y.