https://en.wikipedia.org/wiki/Cloture#:~:text=Cloture%20(UK%3A%20US%3A%20%2F,by%20the%20United%20States%20Senate
"... The Senate's rules originally[34] required a supermajority of two-thirds of all senators present and voting to invoke cloture.[35][36] For example, if all 100 senators voted on a cloture motion, 67 affirmative votes were required to invoke cloture; however, if some senators were absent and only 80 senators voted, only 54 would have to vote in favor.[37] In the early years of the cloture rule, it proved very difficult to achieve this. The Senate tried 11 times between 1927 and 1962 to invoke cloture but failed each time. Filibuster use was particularly heavy by Democratic senators from southern states to block civil rights legislation.[38]
In 1975, the Democratic Senate majority, having achieved a net gain of four seats in the 1974 Senate elections to attain a strength of 61 (with an additional independent senator caucusing with them for a total of 62), reduced the necessary supermajority to three-fifths of senators duly chosen and sworn.[39] In practice, most bills cannot become law without the support of 60 senators.
Under the Senate rules and precedents, certain questions are nondebatable or debate time on them is limited, most notably bills considered under the reconciliation procedure or joint resolutions providing for congressional disapproval. Therefore, these measures cannot be filibustered and are not subjected to the supermajority cloture threshold. Although filing cloture on nondebatable measures is redundant, it has been done on occasion."
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