WASHINGTON — After months of taunts and threats, Republicans and Democrats are preparing to escalate a conflict that each side describes as the "nuclear option": changing the rules for confirming federal judges.

The countdown began yesterday, when members of Congress returned from a two-week spring break. According to some scenarios, it could reach legislative Armageddon by the end of the month.

At issue is whether Senate procedures will be altered so that Democrats can no longer use the most-effective parliamentary weapon in their arsenal — the filibuster — to block votes on controversial nominees to the federal bench. The decision on when, or if, this battle begins rests with Senate Majority Leader Bill Frist, R-Tenn.

Cooler heads on both sides warn of the potential fallout: partisan warfare on a scale not seen since the government shutdown in the mid-1990s.

Democrats say removing the filibuster for judicial nominations would change the fundamental character of the Senate as designed by the Founding Fathers. In retaliation, they have threatened to grind Congress to a halt in all but its most essential functions.

Frist has offered to talk to Minority Leader Harry Reid, D-Nev., about possible compromises, but neither side — or the political interest groups backing them — holds out much hope of a deal to defuse the confrontation.

"No, I don't see any potential compromises," said Nan Aron, president of the Alliance for Justice, a liberal lobbying group. "Judges are too important. They serve for life."

David Keene, chairman of the American Conservative Union, a conservative lobbying group, said: "It's a train bearing down at everybody. Everyone can see it coming, but it's hard to get out of the way, because each side has too much at stake."

Republicans say the fault for the conflict lies with Democrats for having filibustered 10 judges President Bush nominated to the federal bench during his first term.

"The difference now is that these are the first filibusters in history over judicial nominees," said Sen. Orrin Hatch, R-Utah, former chairman of the Judiciary Committee. "That has caused a tremendous amount of animosity."

In fact, in 1968, Senate Republicans used a filibuster to block President Johnson's nomination of Abe Fortas to be Supreme Court chief justice.

Democrats note that the vast majority of Bush's judicial nominees have been confirmed, and say that Republicans have abandoned the long-standing tradition of consulting with the minority party on the more-controversial choices. Judges opposed by the minority party in previous decades often were withdrawn to avoid confrontation.

Democrats were especially angered when Bush took the provocative step earlier this year of renominating seven of the 10 filibustered judges.

"What is driving ... the Senate toward conflict is this White House's efforts to create unnecessary confrontation over judicial nominees," said Sen. Patrick Leahy of Vermont, the ranking Democrat on the Judiciary Committee. "The president insisted on renominating troublesome and divisive choices, rather than working with us to find more consensus nominees who would be fair judges."

The filibuster allows senators to debate as long as they like, which effectively allows them to stall a vote they know they will lose. Originally, a single senator could conduct a filibuster as long as he could keep talking. Current rules make it possible to end one with a vote of 60 senators.

In practice, that means 41 of the chamber's 100 senators can uphold a filibuster indefinitely. There are 44 Democrats in the Senate, and one independent who usually sides with them.

Republicans want to change the rules so that the Senate would need only a simple majority — 51 votes — to end debate and force a vote on a judicial nominee. There are 55 Republicans in the Senate.

Democrats say the filibuster provides protection from "tyranny of the majority." They note that the framers of the Constitution intended the House of Representatives to be run by the majority but specifically designed the Senate as a forum where the minority party and small states would be given greater weight. Taking away the filibuster, the Democrats argue, would fundamentally alter the character of the Senate and, by extension, the balance between majority rule and minority rights enshrined in the Constitution.

"The whole design was to protect minority interests," said Sheldon Goldman, a professor of political science and an expert on judicial history at the University of Massachusetts at Amherst. "So what is being proposed now is of immense historical proportions."

But Republicans argue that Democrats are using the filibuster in a way the framers did not intend: not to block legislation, but to withhold the "advice and consent" that the Constitution requires the Senate to give on presidential nominations to the executive and judicial branches.

"The current Democratic filibusters are not about free speech and extended debate, they are about rejection of nominees with majority support," said C. Boyden Gray, the former White House counsel to President George H.W. Bush who heads the Coalition for Justice, a conservative lobbying group.

"Any senator is entitled to speak all he wants regarding any nominee," Gray said. "But once all voices have been heard, Democrats now want the additional right to block nominees from ever getting a final vote."

As the conflict moves toward its endgame, each side is counting votes. There may be 55 Republicans in the Senate, but a handful — mostly moderates and traditionalists — have expressed concern that the "nuclear option" is too drastic and will damage Republicans the next time they are in the minority.

Material on the 1968 filibuster was provided by The Washington Post.