Senate Democrats said on Wednesday that they would refuse to back a Republican-written stopgap bill to fund the government through Sept. 30, significantly raising the chances of a government shutdown at the end of the week.

After two days of intense closed-door party meetings, Senator Chuck Schumer, Democrat of New York, emerged to say that members of his party could not support the bill approved by the House on Tuesday to keep most federal funding flowing at current levels for the next six months. He instead urged Republicans to pass a monthlong extension to allow time for Congress to consider individual spending bills and reach a compromise that both parties could accept.

“Our caucus is unified” on such a measure to “keep the government open and give Congress time to negotiate bipartisan legislation that can pass,” Mr. Schumer said in a brief statement from the floor.

The announcement left congressional leaders without a clear path to avert a shutdown that would begin at 12:01 a.m. on Saturday should Congress fail to act by then to extend federal funding. Senate Republicans would need the support of at least eight Democrats to overcome procedural hurdles and bring a spending measure to a final vote. Just one, Senator John Fetterman of Pennsylvania, has so far declared he would vote to break any filibuster.

The standoff puts Senate Democrats at risk of being blamed for any shutdown even as they complain about Trump administration disruptions to federal agencies. But they are under pressure from House Democrats and activists to stand against Mr. Trump and Elon Musk as they lead an effort to dismantle broad swaths of the federal bureaucracy, in some cases in direct defiance of Congress, which holds the power of the purse.

With two days left before the shutdown deadline, there is still time for a reversal by Democrats. But most of them have heaped criticism on Republicans’ stopgap spending measure, arguing that it would give Mr. Trump and Mr. Musk too much leeway to continue their unilateral efforts to slash government employees and programs.

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