IN THE 1960s, a handful of southern senators routinely managed to block civil-rights bills. They did this thanks to the filibuster, a Senate curiosity that allows a minority to slow down—or indeed kill—a law by refusing to close off debate. Filibustering senators have been known to recite recipes or read, for hours, from the telephone book.

The only way to stop a filibuster is to muster enough votes to “invoke cloture” and end debate. Today, the majority can shut off debate by getting 60 senators to vote for cloture; until 1975, the threshold was 67 votes. Thanks to the filibuster, the Senate does not operate by majority rule—though there are a few items, such as budget resolutions, that are safe from the device.

The Founding Fathers were mum on the filibuster. The constitution instructs both houses of Congress to adopt their own rules for proceedings; the Senate has chosen to operate by unanimous consent. This means that in principle, barring a cloture rule, a single senator could blather on, bringing the Senate to a halt. The Senate operated without a cloture rule from 1806 to 1917, when it was introduced at the urging of Woodrow Wilson, who was furious that a handful of senators were preventing him from arming American merchantmen against German submarines.

There is a difference between the more common legislative filibuster and the judicial filibuster at the heart of the current showdown. Legislative filibusters are used to force the majority to reshape laws. But a filibuster over a nominee is a zero-sum game: you cannot change the shape of a person. Also, with the legislative filibuster, the Senate is frustrating its own will; with a judicial one, the minority is interfering with two other branches of government, the executive and the judicial.

Before George Bush's election in 2000, only one judicial nominee had ever been successfully filibustered. Ten of Mr Bush's judges were filibustered in his first term. The judicial nominees of his father and Bill Clinton were blocked more often (see chart) but were thwarted by other tactics, such as keeping their nominations buried in committee.

Can the Senate change the rules on the filibuster? For 200 years, the chamber has operated on the understanding that any rule-change requires the vote of two-thirds of its members. The Republicans say this is not a fixed rule; so filibusters can be abolished on a simple majority vote. Others say that means rejecting a 200-year-old tradition of passing down rules and precedents from each Congress to its successor.

If the current debate does indeed “go nuclear”, Dick Cheney, who presides over the Senate, will issue a ruling from the chair to end filibusters of judicial nominees; when the Democrats object, the Republicans will vote down the objection with a simple majority. And then the fireworks will begin.