WASHINGTON — After a rare lashing from conservatives, President Donald Trump declared Thursday he would not sign a bill to keep funding the government because it fails to provide billions for a border wall with Mexico, throwing Congress into deep disarray and risking a federal shutdown this weekend.

Conservatives want to keep fighting for the money to pay for the wall as a last act of the GOP-led Congress before Democrats take House control in January. They warn that “caving” on Trump’s repeated wall promises could hurt his 2020 re-election chances, and other Republicans’ as well.

Trump’s sudden rejection of the legislation, after days of mixed messages, sent Republican leaders scrambling for options back on Capitol Hill.

House Speaker Paul Ryan, exiting a hastily called meeting with Trump and other GOP lawmakers at the White House, said, “We’re going to go back and work on adding border security to this, also keeping the government open, because we do want to see an agreement.”

By afternoon, Trump shifted his terminology, saying he’s not necessarily demanding a border wall but “steel slats” — which is similar to the border security fencing already provided for in the bill.

“I give them a little bit of an out, ‘steel slats,’” Trump said during a farm bill signing at the White House. “We don’t use the word ‘wall’ necessarily, but it has to be something special to do the job.”

The nuance could provide Trump a way to try to proclaim victory. The bill would keep funding at current levels, $1.3 billion, for border security, including pedestrian fencing and replacement fences, not his border wall. It requires only previously used designs.

The House had been set to vote Thursday, before a midnight Friday deadline to fund parts of the government or risk a partial shutdown just before Christmas. The Senate has already approved the bill, which would fund the government to Feb. 8.

House Republicans adopted the last-ditch idea of adding Trump’s $5 billion demand to the bill, along with an $8 billion disaster aid package that many lawmakers support for coastal hurricanes and California wildfires. The plan would be to send the new package to the Senate. GOP Whip Steve Scalise, R-La., said House Republicans would vote late Thursday.

But it was unclear if such a package could pass the House, where some Republicans oppose more border wall funding and others were visibly frustrated by the messy lurch of events. And it would not likely be approved in the Senate because Democrats have already resisted Trump’s $5 billion, as Senate Minority Leader Chuck Schumer call it “ineffective, unnecessary and exorbitantly expensive,” and many senators have already left for the holidays. It would, however, give some House Republicans a release valve for their desire to vote for the wall funding.

Without a deal, more than 800,000 federal workers would face furloughs or be forced to work without pay, disrupting government operations at year-end.

Senate Majority Leader Mitch McConnell, R-Ky., met with Ryan after the House leaders returned from the meeting with Trump. Senators were advised they could be called back to the Capitol Friday for another vote.

Ryan and Majority Leader Kevin McCarthy had endured complaints during a private morning meeting earlier Thursday from rank-and-file Republicans in the Capitol complaining that they were closing out their majority without a fight on a major issue.