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A filibuster is a tactic employed in the United States Senate to prevent a measure from being brought to a vote. The most common form of filibuster occurs when one or more senators attempt to delay or block a vote on a bill by extending debate on the measure. The Senate rules permit a senator, or a series of senators, to speak for as long as they wish, and on any topic they choose, unless "three-fifths of the Senators duly chosen and sworn" (currently 60 out of 100) vote to bring the debate to a close by invoking cloture under Senate Rule XXII.
The ability to block a measure through extended debate was a side effect of an 1806 rule change, and was infrequently used during much of the 19th and 20th centuries. In 1970, the Senate adopted a "two-track" procedure to prevent filibusters from stopping all other Senate business. The minority then felt politically safer in threatening filibusters more regularly, which became normalized over time to the point that 60 votes are now required to end debate on nearly every controversial legislative item.
Efforts to limit the practice include laws that explicitly limit the time for Senate debate, notably the Congressional Budget and Impoundment Control Act of 1974 that created the budget reconciliation process. Changes in 2013 and 2017 now require only a simple majority to invoke cloture on nominations, although most legislation still requires 60 votes.
At times, the "nuclear option" has been proposed to eliminate the 60 vote threshold for certain matters before the Senate. The nuclear option is a parliamentary procedure that allows the Senate to override one of its standing rules, including the 60-vote rule to close debate, by a simple majority (51+ votes or 50 votes with the Vice President casting the tie-breaking vote), rather than the two-thirds supermajority normally required to amend the rules.
One or more senators may still occasionally hold the floor for an extended period, sometimes without the advance knowledge of the Senate leadership. However, these "filibusters" usually result only in brief delays and do not determine outcomes, since the Senate's ability to act ultimately depends upon whether there are sufficient votes to invoke cloture and proceed to a final vote on passage.