<P> After a series of filibusters in the 1960s over civil rights legislation, the Senate put a "two - track system" into place in 1970 under the leadership of Majority Leader Mike Mansfield and Majority Whip Robert Byrd . Before this system was introduced, a filibuster would stop the Senate from moving on to any other legislative activity . Tracking allows the majority leader--with unanimous consent or the agreement of the minority leader--to have more than one bill pending on the floor as unfinished business . Under the two - track system, the Senate can have two or more pieces of legislation pending on the floor simultaneously by designating specific periods during the day when each one will be considered . </P> <P> The notable side effect of this change was that by no longer bringing Senate business to a complete halt, filibusters on particular legislation became politically easier for the minority to sustain . As a result, the number of filibusters began increasing rapidly, eventually leading to the modern era in which an effective supermajority requirement exists to pass legislation, with no practical requirement that the minority party actually hold the floor or extend debate . </P> <P> In 1975, the Senate revised its cloture rule so that three - fifths of sworn senators (60 votes out of 100) could limit debate, except for changing Senate rules which still requires a two - thirds majority of those present and voting to invoke cloture . However, by returning to an absolute number of all Senators (60) rather than a proportion of those present and voting, the change also made any filibusters easier to sustain on the floor by a small number of senators from the minority party without requiring the presence of their minority colleagues . This further reduced the majority's leverage to force an issue through extended debate . </P> <P> The Senate also experimented with a rule that removed the need to speak on the floor in order to filibuster (a "talking filibuster"), thus allowing for "virtual filibusters". Another tactic, the post-cloture filibuster--which used points of order to delay legislation because they were not counted as part of the limited time allowed for debate--was rendered ineffective by a rule change in 1979 . </P>

When did senate rules change to 60 votes