WASHINGTON — Under intense political pressure from his conservative base, President Donald Trump is trying to find a way to fulfill a central promise from his 2016 campaign, constructing a border wall with Mexico, as the specter of a holiday shutdown looms.

In a dramatic showdown, Trump told House Republican leaders Thursday that he will not back a Senate-passed spending measure to avert a partial federal shutdown because it lacks the money he is demanding for the wall. Without a deal, the government will partially shut down Friday at midnight.

Trump’s declaration came amid growing outrage from conservative allies who are urging the president to veto any plan that doesn’t meet his terms. They said Trump would have even less leverage when Democrats take control of the House on Jan. 3 and they worry that Trump’s failure to make good on his signature campaign pledge could hamper his re-election campaign in 2020.

By tweet and official statement, the White House made its position clear.

Trump “does not want to go further without border security,” including the wall, press secretary Sarah Huckabee Sanders said.

Later, House Speaker Paul Ryan, R-Wis., told reporters after a hastily arranged White House meeting that Trump had said he would not “sign the bill that came up from the Senate last evening because of his legitimate concerns for border security.”

Trump complained on Twitter that he “was promised” by congressional leaders that the wall and border security would be “done by end of year (NOW).”

“It didn’t happen! We foolishly fight for Border Security for other countries – but not for our beloved U.S.A. Not good!” he wrote.

He also lashed out at Democrats: “I will not sign any of their legislation, including infrastructure, unless it has perfect Border Security.” He also argued that border security is “tight” due to military and law enforcement efforts.

Throughout Trump’s 2016 campaign, “Build the wall!” was a rallying cry. Trump supporters sought to remind him of that as they pushed him to veto the short-term plan.

On “Fox and Friends” on Thursday, Rep. Mark Meadows, R-N.C., said: “If we’re not going to fight now, when are we going to fight?” The night before, he said that failing to veto the measure would cause “major damage” to Trump’s re-election prospects. “The base will just go crazy,” he said.

The lack of progress on an issue so closely identified with his political ascent may prove costly.

Trump had promised to begin working on an “impenetrable physical wall” along the southern border on his first day in office, but little headway has been made. A March bill included money for 33 miles (53 kilometers) of barrier construction in South Texas’ Rio Grande Valley, but work there has yet to begin. Other work has merely replaced existing barriers that had been deemed “ineffective,” not added miles.

Some of the president’s allies accused him of “caving” on the wall now. They warned of the potential backlash from supporters and the impact it could have on his 2020 re-election effort. The failed promise, they argued, could weaken turnout and leave him more vulnerable to challengers.

Conservative commentator Ann Coulter published a column that called Trump “gutless” and said in a radio interview that she won’t vote for Trump in 2020 if he doesn’t deliver on the wall.

“Nor will, I think, most of his supporters. Why would you?” she asked, arguing that Trump’s time in office will one day go down as “a joke presidency that scammed the American people.”

Trump had said last week that he would be “proud” to have a shutdown to get Congress to approve a down payment on the wall.

Trump had originally demanded $5 billion to begin building the wall this year, but White House officials said earlier this week that he was willing to settle for far less. The temporary measure offers just $1.3 billion for border security fencing and other improvements. That money cannot be used for new wall construction.

Trump has told allies he would be able to go around lawmakers by using the military to fund and carry out construction. Such a move would face significant pushback from Congress as well as legal challenges.

Rep. Adam Smith, D-Wash., the incoming chairman of the House Armed Services Committee, told MSNBC that there has been strong opposition to using Defense Department dollars for border wall construction. And he said that Trump can’t do so without lawmakers’ permission.

The president’s conservative backers insist that Trump should not back down.

“Trump should not sign this bill and leave for Mar-a-Lago, and tell them it’s not gonna get signed and their precious government’s not gonna get back up and running ’til there’s $5 billion,” wrote radio host Rush Limbaugh.

Former Trump campaign adviser Barry Bennett said it was too soon to panic.

“He must have a trick up his sleeve because I can’t imagine he would just walk away from it,” Bennett said.

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