The nuclear or constitutional option is a parliamentary procedure that allows the U.S. Senate to override a rule or precedent by a simple majority of 51 votes, instead of by a super-majority of 60 votes. The presiding officer of the United States Senate rules that the validity of a Senate rule or precedent is a constitutional question. They immediately put the issue to the full Senate, which decides by majority vote. The procedure thus allows the Senate to decide any issue by majority vote, even though the rules of the Senate specify that ending a filibuster requires the consent of 60 senators (out of 100) for legislation, 67 for amending a Senate rule. The name is an analogy to nuclear weapons being the most extreme option in warfare.
In 1917, a threat to use what is now known as the nuclear option resulted in reform of the Senate’s filibuster rules. An opinion written by Vice President Richard Nixon in 1957 concluded that the U.S. Constitution grants the presiding officer the authority to override Senate rules. The option was used to make further rule changes in 1975. In November 2013, Senate Democrats used the nuclear option to eliminate filibusters on executive branch nominations and federal judicial appointments other than those to the Supreme Court.
Before November 2013, Senate rules required a three-fifths vote of the “duly chosen and sworn” members of the Senate – (usually 60 votes) to end debate on a bill, nomination or other proposal; they also require a two-thirds vote (“present and voting” – 67 or more votes) to end debate on a change to the Senate rules. Those rules effectively allowed a minority of the Senate to block a bill or nomination through the technique of the filibuster. This had resulted in a de facto requirement that a nomination have the support of 60 Senators to pass, rather than a majority of 51. A three-fifths vote is still required to end debates on legislation and Supreme Court nominations.
In most proposed variations of the nuclear option, the presiding officer would rule that a simple majority vote is sufficient to end debate. If the ruling is challenged, a majority would be required to overturn it. If the ruling is upheld, it becomes a precedent. This would end what had effectively become a 60-vote requirement for confirmation of an executive or judicial nominee, or the passage of legislation.