Republicans argued that Democrats were "breaking the rules to change the rules" by using a simple majority vote rather than a two-thirds vote to make the rule change. Democrats cited more than a dozen examples of other rule changes that were adopted in such a fashion in the last four decades. They also insisted that Republicans were likely to have made the same change to filibuster rules the next time they gained majority control of the Senate.

Democrats had been threatening to change the rules for nearly a year, but they held off in part because of hesitation by long-serving senators on their side of the aisle. But in recent days, several of the veterans joined recently elected Democrats who led the campaign to change the rules.

"I recognize that I could be back in the minority again, but that's OK if that happens," said Sen. Dianne Feinstein (D-Calif.), who just began a fifth term and who recently decided to support changing the rules. "I want, in the remainder of my five-plus years, to get something done, to be able to get nominees approved, to be able to get bills moved."

While Democrats said the immediate motivation for the rule change was the fight over nominations, the move to end filibusters was probably only a matter of time, reflecting the long-term shift in American politics toward ever-greater polarization, particularly over the last two decades.

Polarization of the two parties has reached levels not seen since the 19th century, according to studies by political scientists Keith T. Poole of the University of Georgia and Howard Rosenthal of New York University. Congress has moved toward a parliamentary system characterized by near-lock-step party voting and deep, consistent ideological differences between the two parties.

Bipartisan coalitions have dwindled, and so have incentives on other side of the aisle to bend the majority party's agenda. For both parties, winning a majority now comes with heavy pressure from constituents to actually adopt the majority's program. That made tools like the filibuster, intended to protect minority-party rights, appear more as an anachronism.

Republicans still have a number of procedural opportunities to delay Senate business, including some that are traditionally dispensed with as a courtesy to allow other issues to come to the floor. McCain, who was key to a deal last summer that defused the last Democratic threat to change filibuster rules, warned of the toxic atmosphere ahead.

"It'll be very poisoned," he said.

michael.memoli@latimes.com

lisa.mascaro@latimes.com

David Lauter in the Washington bureau contributed to this report.