A similar procedure was adopted in the United States Senate in 1917 in response to the actions of isolationist senators who attempted to talk out, or filibuster a bill to arm U.S. merchant ships. President Woodrow Wilson urged the Senate to change its rules to thwart what he called a "little group of willful men", to which the Senate responded by introducing cloture in the form of Rule 22.<1> Cloture was invoked for the first time to end filibuster on the Treaty of Versailles.<2>
This originally required a supermajority of two-thirds of all senators (i.e. 64 out of 96). However, it proved very difficult to achieve this; the Senate tried eleven times between 1927 and 1962 to invoke cloture but failed each time. Filibuster was particularly heavily used by senators from Southern states to block civil rights legislation.
In 1975, the Democratic Senate majority, having achieved a net gain of four seats in the 1974 Senate elections to a strength of 61 (with an additional Independent caucusing with them for a total of 62), reduced the necessary supermajority to three-fifths (60 out of 100). This allowed the Senate majority to invoke cloture, provided that party discipline held. This considerably strengthened the power of the majority, and allowed it to pass many bills that would otherwise have been filibustered. (The Democratic Party had held a two-thirds majority in the 89th Congress of 1965, but regional divisions among Democrats meant that many filibusters were invoked by Southern Democrats against civil rights bills supported by the Northern wing of the party.) Some senators wanted to reduce it to a simple majority (51 out of 100) but this was rejected, as it would greatly diminish the ability of the minority to check the majority.
http://en.wikipedia.org/wiki/Cloturesee also:
Rule 22
The filibuster is related to "cloture," a rule adopted almost 100 years ago requiring a two-thirds vote. At times this was two-thirds of those voting; for a limited time, it was two-thirds of membership.
In 1975, the Senate reduced the number of votes needed to invoke cloture to three-fifths (60) of Senate membership. At the same time, they made the filibuster "invisible" by requiring only that 41 Senators state that they intend to filibuster; critics say this makes the modern filibuster "painless."
http://uspolitics.about.com/od/usgovernment/a/filibuster.htm