Senators prepared for a potentially rancorous day Tuesday _ even by recent standards of partisan unpleasantness _ as Democratic leaders threatened to change procedural rules to stop Republicans from blocking White House nominees for top executive jobs.

Several Senate votes were scheduled to test whether Republicans will allow simple-majority confirmations of a handful of long-stalled nominations. Some senators held out hopes for a breakthrough early Tuesday after one didn't come in a rare, three-hour private closed-door meeting of nearly all 100 senators Monday night.

If neither side retreats, the two parties could be on a collision course, with potentially big ramifications for politics and policymaking for years to come.

Standing alone, the rules change that Majority Leader Harry Reid proposes is limited. It would end the ability of 41 senators, in the 100-person chamber, to use the procedural maneuver known as a filibuster to block action on White House nominations other than judges. The out-of-power party still could use filibuster threats to block legislation and judicial nominees, who seek lifetime appointments.

But critics say Reid's plan would likely prompt Republicans to retaliate by doing even more to reduce the minority party's rights should their party regains control of the Senate. That could happen as early 18 months from now, after the 2014 elections.

"It's a decision that, if they actually go through with it, they will live to regret," Senate Republican Leader Mitch McConnell of Kentucky said of Democrats.

The Senate has a long and bumpy tradition of granting rights to minority-party members that are absent in the 435-seat House of Representatives. The most powerful tool is the filibuster, which essentially kills a measure by using endless debate to prevent a yes-or-no vote.

The mere promise of a filibuster can block Senate action on almost anything unless 60 of the 100 senators vote to overcome it. Filibuster-proof majorities are rare, and Republicans now hold 46 Senate seats.

Both parties have accelerated their use of the filibuster in recent times. Since President Barack Obama took office in January 2009, Republicans have threatened filibusters repeatedly, infuriating Democrats.

Reid said Lyndon B. Johnson faced one filibuster during his six years as Senate majority leader. In the same length of time as majority leader, Reid said, he has faced 413 threatened filibusters. The tactic, he said, blocks action on routine matters that Congress once handled fairly easily.

"The power of an extreme minority now threatens our integrity of this institution," Reid, of Nevada, said in a speech Monday. "My efforts are directed to save the Senate from becoming obsolete."

He called his proposal a "minor change, no big deal." But Republicans, led by McConnell, object bitterly.

Democrats acknowledged that Republicans will turn any such rules change to their advantage if they regain the Senate majority, which the two parties have often swapped in recent decades.

White House spokesman Jay Carney told reporters the Senate "needs to confirm this president's nominees in a timely and efficient manner." That will be true, he said, "for the next president, and the next president after that. This has become ridiculous."

Asked if Obama worries that a filibuster rule change would make the Senate even more dysfunctional, Carney said, "Well, it boggles the mind how they would achieve that."

Senate Republicans particularly object to two union-backed members of the National Labor Relations Board, Richard Griffin and Sharon Block. Obama appointed them when he said the Senate was in recess.

An appeals court said Obama exceeded his authority. The board's actions since the two members took their seats are in legal limbo.

Republicans also have opposed Obama's pick to lead the Consumer Financial Protection Bureau, created in a Wall Street oversight revision that Republicans opposed. Obama nominated his pick, former Ohio Attorney General Richard Cordray, more than two years ago.

Many Republican senators say they will not confirm anyone to the consumer post unless the bureau's leadership structure is changed.

Sen. Richard Durbin of Illinois, the Senate's second-ranking Democrat, said he will not support any changes to the consumer protection bureau's structure as part of a compromise.