During the battles over healthcare and tax reform, Republicans have sent themselves into contortions to abide by the “Budget Reconciliation” rules that allow the majority to get around a Democratic filibuster. So, what is a filibuster? Technically, it is a threat by a minority of senators to debate an issue at length, with the aim of preventing a vote of approval by the majority.

The filibuster is not, as some suggest, required by the Constitution, which simply provides (Article I, Section 5) that each house of Congress “may determine the Rules of its Proceedings.” Senate rules have traditionally allowed unlimited debate, but in 1917, they were amended to allow two-thirds of senators to end debate (“invoke cloture”).

Perhaps the most famous filibuster occurred in 1964, when Southern Democrats spoke around the clock, with cots set up in antechambers to allow naps, in opposition to civil rights legislation. Cloture was eventually invoked and debate ended, and the Civil Rights Act of 1964 was passed.

In 1970, Senate Majority Leader Mike Mansfield sponsored a “two-track” procedure, which allowed senators by unanimous consent or consent of the minority leader, to set aside a filibustered measure and proceed to other matters. The days of round-the-clock filibusters and senators napping on cots came to an end. In 1975, the Senate changed its rules to allow three-fifths of senators to invoke cloture.

Ironically, changes intended to make it easier to stop filibusters led to filibusters being much more common, with cloture being invoked some 1,700 times since 1970. Soon, filibusters became routine, and in time, it became settled that major legislation requires 60 votes — enough to end a filibuster — to pass. This happened although the Constitution requires supermajorities only for ratifying treaties, overriding a presidential veto, and passing an amendment to the Constitution. In 2013, under Majority Leader Harry Reid, Senate Democrats removed the 60-vote requirement for invoking cloture on nominations of executive branch and federal trial and appeals court nominees.

In 2017, to confirm Neal Gorsuch to the Supreme Court, Majority Leader Mitch McConnell expanded the Reid rule change to include Supreme Court nominees. In the background lurks the question of whether a majority of senators will abolish the filibuster for legislation altogether.