The so-called “nuclear option” in the United States senate is a technique which could potentially be used to end a filibuster. Filibusters are an important part of the Senate's rich tradition, and while filibusters are not used frequently, a few have attracted great public attention. With the nuclear option, a simple majority in the Senate could force the issue being filibustered to be tabled, meaning that the Senate would go on with its daily business and return to the issue later.
Before explaining how the nuclear option works, it can help to know what a filibuster is. According to the rules of the United States Senate, nothing limits debate and discussion on an issue being considered by the Senate. A filibuster is used, therefore, to drag out a Senate decision, with a Senator or group of Senators holding the floor with speeches. Typically, a group of Senators works together to hold a filibuster, trading off with each other as they tire out. While the filibuster is carried out, the Senate is not able to go on with its daily business, meaning that it will start to fall behind.
As a result, the decision to filibuster is not taken lightly, and often the members of the Senate will attempt to compromise before the point of filibuster is reached. However, filibusters do happen, and therefore Senators seek out ways to put an end to filibusters. One way to end a filibuster is to invoke cloture, forcing an immediate vote on the issue; if 2/3 of the Senators vote together, the filibuster is ended, and the Senate can carry on with its business.
However, cloture votes aren't always successful, because filibuster often involves a conflict between a very small majority and the minority. As a result, Senators have used the nuclear option several times since the 1950s to end a filibuster.
When the nuclear option is used, a Senator opposed to the filibuster interrupts with a point of order, a request under parliamentary procedure in which the chair is asked to make a ruling to determine whether or not the rules of the Senate have been broken. Because a point of order is meant to remind the Senate of its rules, a point of order can be raised at any time, and the Chair must rule on it immediately. In the case of the nuclear option, the Senator who makes the point of order requests an immediate vote on the issue, and after the Chair rules, another Senator moves to table the issue; since tabling is non-debatable, this forces the Senate to vote, thereby ending the filibuster, and a simple majority decides the outcome of the vote, rather than a 2/3 majority.
Using the nuclear option is extremely controversial, and some people argue that it sets a dangerous precedent. Supporters of the filibuster argue that it is a crucial part of Senate proceedings, ensuring that the Senate is not dominated by a narrow majority, and encouraging cooperation and compromise between Senators on either side of the aisle. By breaking down a filibuster, the majority may bully the Senate into getting what it wants, but it also goes against the fundamental spirit of the Senate.