Let’s go back to day one of the Senate in 1789. Neither the Constitution nor the original rules describe anything approximating a filibuster, in which a single Senator could delay a vote by holding the floor as long as he could stand. In fact, quite the opposite: an original rule specifically allowed for a simple majority to shut off debate. That rule was eliminated in 1806, yet a Brookings Institution study found that no real filibusters took place until the 1830s. Even then, most major legislation passed with simple majority votes, as Senators in the minority understood that to use the filibuster too frequently would invite a rule change.
In 1917, a formal rule was reintroduced to allow debate to be shut off. This time called “cloture,” the rule required a two-thirds majority vote. And yet, according to Senate procedure expert Martin Gold, “between 1917 and 1962, cloture was imposed only five times.” The filibuster frustrated but did not prevent civil rights legislation, and a victorious but irritated majority in 1975 reduced the number of votes required to shut off debate from two-thirds to three-fifths, i.e. the famous 60-vote threshold.
There are two controversial ways to the reduce 60 votes to 51. The first path would change the Senate rules at the beginning of a new session, i.e. every two years. Current Senate rules require a two-thirds majority to change Senate rules, but Senators have debated since 1789 whether previous Senate sessions can bind future Senate sessions. The question has not been definitively resolved because Senators have always tended to renew the rules. But if Senators are unbound, then the Senate could change its rules at the beginning of a new session with only 51 votes, and use that rule change to lower the threshold for every other vote to 51.