The Senate was headed on Friday toward a critical vote to avert a government shutdown at midnight, after Senator Chuck Schumer of New York, the Democratic leader, said he would join Republicans in pushing it past any filibuster by his own party.

A vote to move forward with the G.O.P.-written measure, which would fund the government through Sept. 30, was expected early Friday afternoon. Republicans would need eight Democrats to join them in voting to allow the bill to advance, and Mr. Schumer privately indicated to Democrats on Thursday that he had lined up the votes.

If the Senate fail to pass the funding extension, federal funding would lapse at 12:01 a.m. on Saturday.

Mr. Schumer’s abrupt decision to reverse himself and allow the spending legislation to advance stunned many of his colleagues and angered many Democratic lawmakers and progressive activists. Many in his party have vociferously opposed the temporary spending measure, arguing that it was a capitulation to President Trump, and have said that they would rather shut the government down than vote for it.

As recently as Wednesday, Mr. Schumer was arguing strongly against the bill, instead proposing a monthlong alternative to allow Congress to reach an agreement on individual spending measures with specific instructions over how federal funding should be doled out.

But he reversed course on Thursday, with a shutdown looming and amid concerns that Democrats would be blamed. He argued that a shutdown would only play into the hands of Mr. Trump and Elon Musk, ceding more power to them as they move to defund and dismantle whole swaths of the federal government. In that scenario, he said, the Trump administration could decide which federal workers would be “nonessential” and furloughed. And he warned that Republicans would have little incentive to reopen the government once it shut down.

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