An expected Republican parliamentary maneuver in the U.S. Senate is properly called the “nuclear option.” The drastic measure is intended to assure confirmation of President Bush’s nominees for judgeships by overthrowing a 200-year tradition of unlimited debate. But it could trigger an equally drastic Democratic response that brings most other Senate business to a screeching halt.

The villain of the piece is the filibuster, an extended debate that the Democrats employed in Mr. Bush’s first term to block 10 controversial judicial nominations from an up-or-down Senate vote. They may well use the same strategy again, now that the president has sent over most of the same nominations, as well as against a Supreme Court nominee if a vacancy occurs.

Under present rules, it takes a three-fifths vote of 60 to invoke “cloture” and break a filibuster. The Republicans probably couldn’t make that total, assuming the Democrats hold firm and enough Republican moderates, including Maine Sens. Olympia Snowe and Susan Collins, vote in opposition.

Amending the rules requires an impossible two-thirds vote of 67. So Senate Majority Leader Bill Frist of Tennessee is prepared to employ the nuclear option, although Republicans have renamed it the “constitutional option.” He outlined the strategy in a speech last November before the Federalist Society. Once an extended debate gets under way, a Republican member would raise a point of order, asking the chamber’s presiding officer – probably Vice President Dick Cheney – to rule that filibusters against judicial nominees are unconstitutional. It would take merely a simple majority of 51 votes to uphold the ruling.

If that happens, some Democrats are threatening to block further Senate business by such devices as demanding that every bill on the floor be read word for word, calling for lengthy roll calls on even minor matters, and objecting to calls for the unanimous votes that are used to keep Senate business moving right long. Senate Democratic Leader Harry Reid of Nevada has said that exceptions would be made for support for U.S. troops abroad and for bills that allow the government to operate.

Where are the rights and wrongs in this looming legislative train wreck? The Republicans say the Democrats have abused the filibuster by using it against judicial nominations, on which the Constitution gives the Senate the power to “advise and consent.” The Democrats say Mr. Bush has precipitated the crisis by sending over some of the same right-wing extremists rather than choosing some middle-of-the-road nominees whom the Democrats could accept.

It was Trent Lott, the former Senate Republican leader, who first used the word “nuclear” in describing the strategy. Also, it was the Republicans who in 1968 filibustered President Lyndon B. Johnson’s choice of Supreme Court Justice Abe Fortas to be chief justice until Mr. Johnson finally withdrew the nomination. Democrats say that Republicans blocked dozens of President Bill Clinton’s appellate court nominees.

Still, some members of both parties see the filibuster as a valuable institution in maintaining the Senate as a deliberative body and protecting the interests of whichever party is in the minority.

Cooler heads may prevail, but the better guess is a head-on collision, driven by conservatives to move the entire court system to the right, a president who is confident that he has a mandate for change, and, incidentally, a Senate Republican leader who has an unswerving eye on his own candidacy for the presidency in 2008.