Senate Republicans on Thursday bulldozed past Senate precedents and changed the rules to break a Democratic blockade of President Trump’s nominees, in an extraordinary move that is likely to undercut Congress’s future role in vetting executive branch officials.

The change, pushed through along party lines, lowered the existing 60-vote threshold for considering a group of presidential nominees to a simple majority, weakening the ability of individual senators to block nominees they find objectionable. It was the latest step in a yearslong back-and-forth between the two parties that has eroded the filibuster, a once-potent Senate tool to protect the rights of the minority and force consensus.

In this case, Republicans resorted to the move in an effort to steer around Democratic obstruction of Mr. Trump’s nominees, which has created a backlog of more than 100, angering the president and frustrating his allies in Congress. They framed it as a necessary step to grease the wheels of a chamber mired in partisan rancor and to return the Senate to its longstanding norm of confirming lower-level nominees without individual votes.

“For two centuries, most presidential nominees have sailed through this chamber by voice vote and by unanimous consent,” said Senator John Barrasso of Wyoming, the No. 2 Republican. “That was the gold standard of advice and consent. Senator Schumer and Senate Democrats abandoned it.”

Democrats, vocally opposed to Mr. Trump’s policies and tactics, have demanded that every position be subject to individual consideration, delaying the approval of the president’s nominees and cluttering the Senate schedule. Senator Chuck Schumer of New York, the minority leader, said the change would do irreparable damage to the Senate and its constitutional prerogatives, rendering it “a conveyor belt for unqualified Trump nominees.”

“We are supposed to debate and take votes on nominees, especially when the executive branch is grossly breaking norms by sending us woefully unqualified, unscrupulous, and in some cases deeply dishonest individuals for powerful and important positions,” he said.

The rule change, which is expected to be finalized next week after a series of procedural votes, will allow the Senate to confirm an unlimited number of lower-level executive branch nominations at a time without the threat of a filibuster. That means Republicans will no longer be forced to negotiate with Democrats to agree on which nominees are deserving of speedy confirmation, as the majority currently must do, but will instead be able to steer around any opposition and push through dozens with a single vote.

Senator John Thune, Republican of South Dakota and the majority leader, began the process on Monday by introducing 48 of Mr. Trump’s nominees together to allow them to be confirmed as a group.

That bloc covers a variety of positions, such as assistant secretaries in several cabinet departments, the inspector general at the C.I.A. and ambassadors, including Kimberly Guilfoyle, the former fiancée of Mr. Trump’s eldest son whom the president chose to be the ambassador to Greece.

The breadth of the list illustrates the far-reaching consequences of the rules change. Previously, senators needed 60 votes to bring a bloc of nominations to the floor, all but guaranteeing that both parties would haggle over who might be included.

Now, the party in power will have the ability to unilaterally approve dozens of executive branch nominees with little resistance. And while nominees must still win approval from Senate committees, there will be even less incentive for presidents to choose people who can draw bipartisan support, since members of the minority party will be presented with even fewer opportunities to delay or block anyone’s confirmation.

Republicans, who currently hold 53 seats in the Senate, pushed through the rules change on a 53-to-45 vote, turning to a tactic known as the “nuclear option” to set a new precedent that overrules what has been done in the past.

The dramatic name refers both to the extreme nature of the move and its assurance of mutual destruction: Republicans and Democrats alike must suffer the consequences when they find themselves in the minority.

It was once seen as so devastating to the Senate’s spirit of bipartisan compromise that leaders often threatened it but rarely followed through.

There were signs on Thursday that such concerns remained alive on Capitol Hill. Even after the Senate began voting on Mr. Thune’s initial step to begin the rule change, a bipartisan group of senators continued to haggle over an agreement to avoid the nuclear option, holding open the vote for much of the afternoon while they engaged in frenzied talks.

Senator Sheldon Whitehouse, Democrat of Rhode Island, said he believed there was a “considerable, good-faith desire to come to a rules change that is bipartisan and agreeable to most all of us.” He urged senators to cooperate to prove they could turn down the political temperature in the wake of the shooting death of the right-wing activist Charlie Kirk.

The proposal that senators were discussing would have capped the number of nominations that could be grouped together at 15. But those negotiating the deal would have needed the consent of all 100 senators to move forward, and while Democrats sought to work into the weekend to persuade their colleagues, Republicans were skeptical that a deal could be reached and instead moved ahead unilaterally.

“We were achingly close to a deal,” Senator Brian Schatz, Democrat of Hawaii, said on the Senate floor. “But I am afraid that my colleagues on the other side of the aisle have run out of patience.”

But Mr. Thune, visibly frustrated, argued that Democrats had had months to come up with a deal to end their roadblock on nominees. “How much time is enough?” he said, addressing Mr. Schatz.

Currently, more than 1,000 executive branch positions require Senate confirmation. Traditionally, many picks below the cabinet level have been confirmed through voice votes or by unanimous consent, essentially an agreement by all senators to bypass debate. But senators in the minority retained the ability to block an individual nominee they found objectionable, slowing the process and allowing more time for the Senate to scrutinize them.

“We are steadily handing away most of the significant constitutional powers of the Senate,” said Senator Chris Coons, Democrat of Delaware. “That is very dangerous. What happened today with a narrow Republican majority to change the rules of the Senate and make it easier to confirm dozens and dozens of nominations just weakens our Constitution.”

Even with the rules change, some limits will remain. The new Senate precedent will not allow for group confirmations that include judicial appointments or high-profile jobs such as cabinet secretaries.

It does allow for group confirmations of federal prosecutors, at a time when Mr. Trump has pushed to install loyalists and erase the Justice Department’s independence from the White House.

Currently, another longstanding Senate tradition, the “blue slip,” allows home-state senators to essentially veto nominees for U.S. attorney in their home states. But Mr. Trump has demanded that Republicans abandon that custom, raising fears that it, too, could vanish in the face of a highly polarized political atmosphere.

That increased partisanship has seized the Senate, prompting party leaders to make rules changes that were once viewed as beyond the pale.

In 2013, Democrats lowered the filibuster threshold on most nominees to a simple majority, out of mounting frustration that Republicans were blocking Obama administration appointees to an influential court.

Four years later, Republicans retaliated by lowering the threshold for Supreme Court nominees. That move allowed Mr. Trump to put three justices on the bench during his first term.

Megan Mineiro contributed reporting.