The changes negotiated by Senate Majority Leader Harry Reid, D-Nev., and Minority Leader Mitch McConnell, R-Ky., would reduce the number of times opponents may use filibusters, the delaying tactics that minority parties in the chamber long have used to kill legislation, but not eliminate them. It would also limit the time spent debating some bills and nominations.
Ironically, votes to streamline the rules were held in the evening of a long, painfully typical day for the chamber that featured a sprinkling of senators' speeches and long period when the Senate chamber idled with nothing happening. Off the floor, private negotiations were held nailing down the agreement's final details.
The pact does not represent a dramatic reworking of Senate rules and leaves the minority party with far more power than it has in the House, where procedures allow a united majority party to muscle through its priorities.
But it would streamline some of the Senate's work and avoid what could have been prolonged, nasty battling between the two parties if Democrats - frustrated by the GOP's growing reliance on the delays - tried ramming through more decisive changes.
In an irony that underscores the Senate's complex rules, it was expected to take the chamber two votes to approve the changes.
The curbs on filibusters fall short of what Reid initially said he favored months ago. He wanted to completely ban the tactic's use when the Senate tries to begin debating a measure, and he threatened to use Democrats' strength in the Senate to enact that change and perhaps others by a simple majority vote.
That tactic is called the "nuclear option" because of the bitter partisan warfare it would likely trigger in the chamber, potentially halting almost any business the Senate tried to conduct.
Typically, rules changes take a two-thirds majority.
The restrictions also fall far short of what some of the Senate's newer Democrats were demanding.
Their proposals included requiring filibustering senators to actually debate on the chamber's floor, a practice immortalized in the film "Mr. Smith Goes to Washington" but seldom used in recent years. Instead, most filibusters in recent decades occur when a senator simply informs majority Democrats that they will need the votes of 60 of the 100 senators to end delaying tactics.
The No. 2 Senate Democratic leader, Richard Durbin of Illinois, said this week that Democrats lacked enough votes to force that proposal through the Senate.
Durbin said Thursday that the tentative deal was "great for the Senate," and said lawmakers who wanted tighter curbs would have to settle for less.
"That's how this world works," Durbin told reporters. "People start aspiring at very high levels, then you get a negotiation, then you reach something called compromise. And I think we are at that point."
Sen. Roy Blunt, R-Mo., said that while he and many other Republicans didn't believe the changes were needed, "This does this without doing irreparable damage to the Senate as an institution."
Sen. John McCain, R-Ariz., among a bipartisan group of eight senators who proposed changes similar to those embraced in the deal, said the agreement showed that cooperation across party lines remained possible.
Alluding to the partisan clashes that could have resulted if Democrats used the "nuclear option," McCain said, "It could have really had repercussions for a very long time here in the Senate."
Tight restraints on filibusters were championed by less-senior Democrats like Sens. Jeff Merkley, D-Ore., and Tom Udall, D-N.M. They are frustrated with the chamber's often glacial debates and the ability of the minority - these days Republicans - to kill bills with less than majority support.
"Are they everything I want? Of course not," Udall said in an interview. But he said the Senate is "moving in the right direction. With these changes, it will make this a more efficient institution."
In an email to supporters, Merkley called the agreement an improvement but also complained that senators delaying legislation would not have to appear on the Senate floor.
"If senators want to force more debate, they should make their case before the American people so the public can weigh in on whether the filibusterers are heroes or bums," Merkley wrote.
More veteran Democrats like Sen. Carl Levin of Michigan were reluctant to stifle the tactic, mindful that his party could find itself in the minority after any election and would want to be able to use the maneuvers.
The liberal group Common Cause, which has filed a lawsuit challenging the constitutionality of the filibuster, criticized Reid for the agreement, saying the senator "has gone missing in the fight for filibuster reform."
Democrats say Republican use of the tactic has become almost routine and far too frequent. Republicans say they use it because Reid often blocks them from offering amendments.
As part of the agreement, filibusters could be avoided when the Senate tries beginning debate on a bill or nomination. In return, the majority leader would have to allow each party to offer at least two amendments - addressing a major complaint of Republicans that their amendments are often shut out.
In addition, once the Senate votes to limit debate on certain nominations - district court judges and administration posts below Cabinet level - the debate would be limited to two hours, far below the 30 hours now allowed. The proposal was aimed at speeding the time spent on such nominations.
In addition, instead of three separate opportunities for opponents of a bill to wage filibusters to block a Senate vote allowing the chamber to try writing compromise legislation with the House, there would only be one such filibuster allowed.
According to the Senate Historian's Office, there were 73 "cloture" votes to end filibusters in the two-year Congress that ended earlier this month. There were 91 such votes in the Congress that served in the two previous years, and 112 in the two-year Congress before that. Republicans were the Senate minority party in each of those Congresses.
Those are the three highest number of cloture votes in any Congress since the Senate started allowing such votes to end filibusters nearly 100 years ago.