Although it is not provided for in the formal rules of the Senate, the threat of the nuclear option has played a key role in filibuster reform historically.[9] A series of votes in 1975 have been cited as a precedent, although some of these were reconsidered shortly thereafter.

The maneuver was brought to prominence in 2005 when Majority Leader Bill Frist (Republican of Tennessee) threatened its use to end Democratic-led filibusters of judicial nominees submitted by President George W. Bush. In response to this threat, Democrats threatened to shut down the Senate and prevent consideration of all routine and legislative Senate business. The ultimate confrontation was prevented by the Gang of 14, a group of seven Democratic and seven Republican Senators, all of whom agreed to oppose the nuclear option and oppose filibusters of judicial nominees, except in extraordinary circumstances.

http://en.wikipedia.org/wiki/Nuclear_option