The 1917 version of Rule 22 required an affirmative vote of two-thirds of senators present and voting to end debate.
Notes on verification
Multiple authoritative sources (Senate.gov, CRS, Brookings, National Constitution Center, National Geographic) confirm the 1917 Rule 22 cloture threshold as two-thirds present and voting.
Sources
- Filibuster, cloture: US Senate rules featured in abortion debate | Politics News | Al Jazeera (aljazeera.com)
- https://www.nationalgeographic.com/history/article/origins-of-filibuster-united-states-senate (nationalgeographic.com)
- https://www.senate.gov/about/powers-procedures/filibusters-cloture/overview.htm (senate.gov)
- https://www.senate.gov/about/powers-procedures/filibusters-cloture.htm (senate.gov)
- https://www.everycrsreport.com/reports/RL32149.html (everycrsreport.com)
- https://constitutioncenter.org/blog/filibustering-in-the-modern-senate&lang=en (constitutioncenter.org)
- https://www.brookings.edu/articles/what-is-the-senate-filibuster-and-what-would-it-take-to-eliminate-it/ (brookings.edu)
- https://www.npr.org/2022/01/17/1072714887/filibuster-explained (npr.org)