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 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 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 when the GOP 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.
Leaving Monday night's meeting, Sen. John Boozman, R-Ark., said, "I think it's going to come to a head tomorrow." The two parties need a breakthrough, he said, but "it's not there yet."
Unlike the 435-member House, the Senate has a long and bumpy tradition of granting rights to minority-party members. 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."
This notion that things can't get much worse in the often stalemated Senate seems to have convinced numerous senators and interest groups in recent months that there's little risk in changing traditions to end at least some of the logjams.
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.
The administration and Senate leaders have discussed the possibility of replacing Griffin and Block with new nominees, but they reached no accord. Sen. Tom Harkin, D-Iowa, vowed to cause a new ruckus if the two are replaced.
"I think it will be grossly unfair to throw them out, simply to make a deal, when they've done nothing wrong," he said late Monday.
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.
Several senators said Cordray's situation is easier to resolve than the NLRB controversy. But Sen. Richard Durbin of Illinois, the Senate's second-ranking leader, said he will not support any changes to the consumer protection bureau's structure as part of a compromise.
Republicans seem to have dropped efforts to block Gina McCarthy to head the Environmental Protection Agency. Reid, however, accused them of unwarranted delaying tactics, which included 1,100 written questions to the nominee.
Thirteen people were arrested after multiple acts of vandalism and several assaults in Los Angeles' Crenshaw District, Mayor Eric Garcetti and Police Chief Charlie Beck said at a news conference.
Garcetti and Beck didn't elaborate on the assaults or any injuries, but at least one man could be seen in the street with a head injury.
More than 300 officers were called to the scene and were at first slow to directly engage protesters in an attempt to allow a peaceful end to the demonstration, Beck said. But the chief said police would take a much stricter posture in the coming nights.
"This will not be allowed to continue," Beck said.
Several hundred mostly peaceful protesters gathered Monday night at Leimert Park southwest of downtown LA, many of them chanting, praying and singing.
But a smaller group of between 100 and 150 people splintered off and began blocking traffic on nearby Crenshaw Boulevard, some of them jumping on cars and breaking windows at liquor stores and fast food outlets.
Several protesters ran into a Wal-Mart store, where they knocked down displays before store security chased them out, and police began guarding the door.
Tonya Williams was shopping with her daughter when the protesters burst in and security briefly locked down the store.
"We thought we were going to be stuck in there," Williams said. "We saw the merchandise all thrown around. They had pulled the rack down, and there was merchandise all over the floor."
TV news helicopters showed some people kicking and punching others along the street, including two people sitting on a bus bench.
Police declared the gathering an unlawful assembly about three hours after it began, and most of the crowd left the street.
Garcetti, who returned early from an East Coast trip because of the demonstrations, praised the "overwhelming majority" who protested peacefully.
"We are a better city than what we have seen tonight in the hands of a few people," the mayor said.
In Oakland, dozens of demonstrators briefly blocked all lanes of Interstate 880 at the tail end of rush hour, stopping traffic in both directions for several minutes before lanes were cleared by authorities. Several protesters laid their bicycles on the ground in front of stopped cars.
"You've got to go. You will go to jail," one police officer shouted at demonstrators who were blocking traffic, the Oakland Tribune reported. However, police decided not to make arrests as the marchers, chanting "Justice for Trayvon Martin," were directed back to surface streets.
Later, another group tried to march up the onramp to Interstate 580 before being turned away by Oakland police and California Highway Patrol officers.
The freeway protesters broke off from a larger group organized via social media that gathered at Oakland City Hall about an hour earlier.
More than a half-dozen people were arrested on charges of vandalism and assault, both felony and misdemeanor, Oakland police spokeswoman Johnna Watson said.
Over the weekend, demonstrators in Oakland and Los Angeles blocked traffic and clashed with police in protests over a Florida jury's acquittal of neighborhood watch volunteer Zimmerman in the shooting death of the unarmed black teenager.
Police shot beanbag rounds and arrested six people - including one on suspicion of assaulting an officer - while breaking up relatively small demonstrations before dawn.
No injuries were reported to either demonstrators or officers.
Most demonstrations around the state were peaceful.
Democratic Sen. Barbara Boxer sent a letter Monday to U.S. Attorney General Eric Holder supporting the Justice Department decision to review the case to determine whether Martin's civil rights were violated.
"I respect the fact that the jury has spoken ... but I don't think this should be the last word," Boxer wrote in the letter.
CAIRO (AP) - A senior Health Ministry official says clashes overnight between police and supporters of Egypt's ousted president have left at least seven people dead.
Khaled el-Khateib also says 261 people were injured in the violence that broke out late Monday and carried on into the early morning hours of Tuesday in four different locations in the capital, Cairo.
Thousands of supporters of Mohammed Morsi, who was overthrown by the military, were protesting to press their demands that Morsi be reinstated as president.
Egypt's military deposed Morsi on July 3 after days of mass street protests calling for him to step down.
The ousted president's supporters say he was ousted by a military coup that overturned democratic rule.