In 1975, the US Senate voted to lower the cloture threshold to three-fifths of senators serving overall, establishing the current 60-vote requirement.
Notes on verification
Confirmed by Senate's own official history pages and multiple independent academic/legal sources detailing the 1975 rule change from two-thirds to three-fifths of senators duly chosen and sworn (60 votes).
Sources
- The federal shutdown and the Senate filibuster: What could happen now? | Donald Trump News | Al Jazeera (aljazeera.com)
- https://www.senate.gov/about/powers-procedures/filibusters-cloture.htm (senate.gov)
- https://www.senate.gov/artandhistory/history/common/generic/Origins_Cloture.htm (senate.gov)
- https://mondale-librarycollections.law.umn.edu/filibuster-cloture (mondale-librarycollections.law.umn.edu)
- https://www.senate.gov/about/powers-procedures.htm (senate.gov)
- https://bipartisanpolicy.org/explainer/senate-filibuster-explained/ (bipartisanpolicy.org)