Senator Chuck Schumer of New York, the minority leader, broke with his party on Thursday and lined up enough Democrats to advance a Republican-written bill to keep federal funding flowing past a midnight Friday deadline, arguing that Democrats could not allow a government shutdown that many of them have demanded.
During a private luncheon with Democrats, Mr. Schumer stunned many of his colleagues by announcing that he planned to vote to allow the G.O.P. bill to move forward, and indicated that he had enough votes to help Republicans break any filibuster by his own party against the measure, according to attendees and people familiar with the discussion. It was a remarkable move at a time when many Democrats in both chambers and party activists have been agitating vocally for senators to block the measure in defiance of President Trump.
In a speech hours later on the Senate floor, Mr. Schumer announced his plan to vote to move forward with the Republican measure, which would fund the government through Sept. 30. He argued that if Democrats stood in the way, it would lead to a shutdown that would only further empower Mr. Trump and Elon Musk in their bid to defund and dismantle federal programs.
“The Republican bill is a terrible option,” Mr. Schumer said in his evening speech. “It is deeply partisan. It doesn’t address far too many of this country’s needs. But I believe allowing Donald Trump to take even much more power via a government shutdown is a far worse option.”
In a shutdown, Mr. Schumer said, “the Trump administration would have full authority to deem whole agencies, programs and personnel nonessential, furloughing staff with no promise that would ever be rehired.”
And he warned that if the government closed, Mr. Trump and Republicans would have no incentive to reopen it, since they could selectively fund “their favorite departments and agencies, while leaving other vital services that they don’t like to languish.”
His announcement came little more than 24 hours before a shutdown deadline. If Congress fails to approve legislation extending federal funding, it will lapse at 12:01 a.m. on Saturday.
In a meeting with reporters after his remarks, Mr. Schumer declined to confirm that he had sufficient Democratic votes to move the legislation past procedural hurdles, saying that senators were making their own decisions. But other Democrats said they were confident that he had the votes.
Senate Republicans are expected to need the support of at least eight Democrats to steer around a filibuster. Other than Mr. Schumer, only one Democrat, Senator John Fetterman of Pennsylvania, has said he would support the bill.
Mr. Schumer has long seen responsibility for government shutdowns as a political albatross. But many Democrats on Capitol Hill have refused to go along with the stopgap spending measure, regarding it as their only leverage against Mr. Trump. All but one House Democrat voted against the plan on Tuesday, and many of them, along with their colleagues in the Senate, have spent the last few days pressing to hold firm against it to challenge the president.
In lengthy closed-door group discussions over the past three days, Senate Democrats have agonized over how to handle the spending bill, which would keep government funding largely flat over the next six months.
Many of them described an impossible choice between two evils: supporting a bill that would give the Trump administration wide latitude to continue its unilateral efforts to slash government employees and programs, or a shutdown that would also give Mr. Trump and his team broad leeway to decide what to fund.
Several Democrats — including both centrists and progressives — declared that they could not back legislation that would give that kind of power to the president and Mr. Musk. They groused that Republicans had unilaterally drafted the legislation and refused to consider any changes to win their votes, essentially daring them to take the blame for a politically toxic shutdown.
“What everyone is wrestling with is that either outcome is terrible,” said Senator Martin Heinrich of New Mexico. “This president has put us in a position where, in either direction, lots of people’s constituents are going to get hurt and hurt badly. So people are wrestling with what is the least worst outcome.”