Bill Frist and the Republicans considered making this change back in 2005 but
didn’t. They were trying to get President Bush’s nominees approved.
https://www.google.com/#q=2005+bill+frist+explores+nuclear+option
Originally both the House and Senate had unlimited debate. The House later implemented
limited debate as the number of members grew.
In 1917 the Senate adopted Rule 22 limiting debate with the consent of 67 members. In
1975 that was changed to the consent of 60 members. Now the number is a majority
for some items and the 60 remains for other items.
Standing Rules of the Senate:
http://www.rules.senate.gov/public/index.cfm?p=RulesOfSenateHome