Filibuster in the United States Senate

A filibuster in the United States Senate is a dilatory or obstructive tactic used in the United States Senate to prevent a measure from being brought to a vote. The most common form of filibuster occurs when a senator attempts to delay or entirely prevent a vote on a bill by extending the debate on the measure, but other dilatory tactics exist. The 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" (usually 60 out of 100 senators) brings debate to a close by invoking cloture under Senate Rule XXII.

According to the Supreme Court ruling in United States v. Ballin (1892), changes to Senate rules could be achieved by a simple majority. Nevertheless, under current Senate rules, a rule change itself could be filibustered, with the votes of two-thirds of those senators present and voting (as opposed to the normal three-fifths of those sworn) needed to end debate. Despite this written requirement, the possibility exists that the Senate's presiding officer could on motion declare a Senate rule unconstitutional, which decision can be upheld by a simple majority vote of the Senate.