WASHINGTON — Senate Republicans changed longstanding rules on Thursday to clear the way for the confirmation of Judge Neil M. Gorsuch to serve on the Supreme Court, bypassing a precedent-breaking Democratic filibuster by allowing the nomination to go forward on a simple majority vote.

In deploying the so-called nuclear option, lawmakers are fundamentally altering the way the Senate handles one of its most significant duties — a sign of the body's creeping rancor in recent years after decades of at least relative bipartisanship on Supreme Court matters. Both parties have likewise warned of sweeping effects on the future of the court, predicting that the shift will lead to the elevation of more ideologically extreme judges if only a majority is required for confirmation.

Senate Democrats in 2013 first changed the rules of the Senate to block Republican filibusters of presidential nominees to lower courts and to government positions, but they left the filibuster in place for Supreme Court nominees, an acknowledgment of the sacrosanct nature of the high court. That last pillar was knocked down on a party-line vote, with all 52 Republicans voting to overrule Senate precedent and all 48 Democrats and liberal-leaning independents voting to keep it.

The Senate then voted 55-45 to cut off debate — four votes more than needed under the new rules — and move to a final vote on Judge Gorsuch's confirmation Friday evening, with a simple majority needed for approval.

Lawmakers first convened late Thursday morning to decide whether to end debate and advance to a final vote on Judge Gorsuch. Republicans needed 60 votes — at least eight Democrats and independents joining the 52-seat majority — to end debate on the nomination and proceed to a final vote. Only a handful of Democrats defected, and the vote failed, 55-45, leaving Republicans to choose between allowing the president's nominee to fail or bulldozing long-held Senate practice.

For weeks, the outcome of the Senate fight has appeared preordained, even as members lamented its inevitability as a low moment for the chamber. In recent days, faint rumblings of a deal to avert the clash had faded almost entirely.

Republicans have argued that changing the rules to push through the nomination was their only option, seeking to shift responsibility for blowing up the Senate's longstanding practices to the Democrats. Allowing the filibuster to succeed, they said, would cause more damage than overriding Senate precedent to ensure it fails.

"This is the latest escalation in the left's never-ending judicial war, the most audacious yet," Senator Mitch McConnell of Kentucky, the majority leader, said after describing Democratic opposition in the past to Judge Robert H. Bork and Justice Clarence Thomas. "And it cannot and it will not stand. There cannot be two sets of standards: one for the nominees of the Democratic president and another for the nominee of a Republican president."

But Democrats had shown no signs of forsaking their filibuster plans all week calling for Judge Gorsuch to be replaced by a compromise nominee if he could not earn 60 votes. Their stance has pleased their most progressive voters, who have preached resistance to President Trump at every opportunity, and supplied the minority party with perhaps its loudest megaphone so far under the new president.

Many Democrats remain furious over the treatment of Judge Merrick B. Garland, President Barack Obama's nominee for the seat left vacant with the February 2016 death of Justice Antonin Scalia. Republicans refused to even consider Judge Garland during the presidential election year, a fact Mr. McConnell has not dwelled on during public statements about the history of Republican behavior under Democratic presidents.

"There must have been a hacking into his computer," Senator Richard J. Durbin, Democrat of Illinois, said of Mr. McConnell on Thursday from the Senate floor, "because he can't print the name Merrick Garland to include in the speech."

At the same time, critics of Judge Gorsuch say they have identified ample reasons to oppose him, chafing at the suggestion that Democrats are merely seeking payback. They have cited concerns over Judge Gorsuch's record on workers' rights and whether he will be reliably independent from Mr. Trump and conservative groups like the Federalist Society, among other issues.

"The more we learned about Judge Gorsuch's record, the more we didn't like," said Senator Chuck Schumer of New York, the Democratic leader.

Republicans have likewise taken particular pains to highlight what they deem escalations of hostilities by Democrats through the years. These include efforts to block judicial nominees under President George W. Bush and the 2013 rule change, when Democrats barred other filibusters.

For the body's most veteran lawmakers in both parties, the moment has been trying.

The longest-serving senator, Patrick J. Leahy, Democrat of Vermont, had initially said he was disinclined to embrace a filibuster. But on Wednesday, he railed against his Republican colleagues from the Senate floor.

"They have no interest in playing by the rules," he said. "They prefer to break them."

Senator Orrin G. Hatch of Utah, the longest-serving Republican, said the episode had bothered him "as much as any battle that we've had" through his four decades in the chamber.

But when considering a judge whom conservatives view as deeply qualified and uncontroversial, Mr. Hatch said he had no choice. The Supreme Court filibuster would have to go.

"I won't be happy about that," he said. "But I will do it."