While the time allowed for debate of a bill is limited in the House of Representatives, the Senate does not limit debate unless by a unanimous consent agreement. These agreements, entered into by the majority leader or floor manager of a bill, set the grounds for the length of time a bill will be debated, who will control the floor, and possibly what kinds of amendments will be allowed. The Senate's no-limits policy has sometimes led to a filibuster, a practice in the Senate of "talking a bill to death" aimed at delaying or preventing legislative action. A minority of senators (or just one) may delay or prevent legislative action by holding the floor and preventing other senators from speaking. The filibuster enables the minority party to prevent the majority party from quickly debating and voting on a bill. Although the practice is well ingrained in the Senate, it is only a custom and not part of the Constitution.
During a filibuster, senators may speak for hours on end, and not necessarily about the legislation at hand. For example, in 1957 Senator J. Strom Thurmond spoke for more than 24 hours to prevent a vote on that year's Civil Rights Act from taking place. A few years later, a 60-day filibuster took place to block action on the Civil Rights Act of 1964.
Filibusters can be forced to a close, however. In 1917 the Senate adopted Rule 22, which established the rule of cloture, permitting the Senate to end debate on a bill with a three-fifths majority vote. When Rule 22 was originally passed, it allowed the Senate to vote to limit debate with a two-thirds majority vote. This percentage turned out to be difficult to achieve. After only 4 of 23 cloture movements were successfully voted on between 1919 and 1960, the Senate amended Rule 22 to require a three-fifths majority vote.
In the 21st century, Senate majority parties have made modifications to the filibuster. Some members of Congress threatened to remove the option of the filibuster entirely by changing the rules to allow only a simple majority vote to end debate. In 2013 Democrats voted along party lines to enact the so-called "nuclear option"—changing the rules to allow for a simple majority, or 51, votes to approve presidential cabinet nominees and nominees for judgeships below the Supreme Court. In 2017 the Republican majority changed the rule to include Supreme Court nominees.