US President Barack Obama has lent his support to a move by Senate Democrats to limit Republicans' ability to block White House nominations.
The measure, known as the "nuclear option", curbs the power of the upper chamber's minority party to use a blocking tactic known as a filibuster.
Mr Obama cited Republicans' "unprecedented pattern of obstruction".
Republicans vowed they would use the new rule against Democrats if they won back the Senate in the next election.
Correspondents said Thursday's motion, which passed 52-48, would make American politics even more acrimonious.
At the White House on Thursday, Mr Obama conceded that neither party had been blameless in filibusters.
But he said it was unacceptable that nearly 30 of his nominees had been blocked from confirmation since he took office.
The US leader called it an "unprecedented pattern of obstruction in Congress" to block qualified candidates.
"A deliberate and determined effort to obstruct everything, no matter what the merits, just to refight the results of an election, is not normal," he said.
Under the rule change, the party in control of the upper chamber of the US Congress now requires just 51 votes, instead of 60, to overcome a filibuster of White House judicial or executive nominations.
Democrats currently have 55 seats in the Senate; Republicans 45.
The motion does not affect Supreme Court nominations or regular legislation.
It is viewed as the most significant rule change since 1975, when the requirement to end any filibuster on the Senate floor was decreased from two-thirds of all senators to the current 60.
"This is a power grab," argued Senator Lamar Alexander. "It's another raw exercise of political power to permit the majority to do anything it wants whenever it wants to do it."
Senate Majority Leader Harry Reid and his fellow Democrats - who control the chamber for now - had repeatedly warned they might ram through the motion.
In July, Democrats set aside a threat to do so only after Republicans dropped their long-standing objections to several White House nominees.
Earlier this week, Senate Republicans filibustered the nomination of a judge to a key appeals court, the third such move since October. Democrats could not muster enough votes to overcome the manoeuvre.
Senate Minority Leader Mitch McConnell said the Democrats had themselves used the filibuster during the two terms of President George W Bush.
Democrats used the filibuster tactic 38 times during Mr Bush's two terms, while Republicans used it 54 times in Mr Obama's first term, according to a June report from the nonpartisan Congressional Research Service.
"You may regret this a lot sooner than you think," Mr McConnell, the Senate's top Republican, said on the chamber floor.
Republican Senator Chuck Grassley told the Washington Post earlier that if the motion passed, his party would retaliate if it regained control of the chamber.
He said they would change the rules again to block any Democratic filibusters of Republican Supreme Court nominees.