WASHINGTON -- Breaking ranks with Democratic leaders, Senators John F. Kerry and Edward M. Kennedy yesterday vowed to lead a filibuster against the Supreme Court nomination of Samuel A. Alito Jr., forcing their colleagues to decide whether to join a high-stakes -- but likely fruitless -- attempt to use Senate rules to block the judge's confirmation.
The move by the two Massachusetts Democrats intensifies pressure on other Democratic senators, who have been torn between appeasing their abortion-rights supporters and risking condemnation for attempting to block a vote on a judge with impressive credentials and majority support in the Senate.
''Judge Alito's confirmation would be an ideological coup on the Supreme Court," Kerry said. ''We can't afford to see the court's swing vote, Justice Sandra Day O'Connor, replaced with a far-right ideologue like Samuel Alito."
The Alito nomination has turned into a nightmare for Democratic leaders, who have almost uniformly opposed Alito, but resisted taking the more radical -- and hotly disputed -- step of mounting a filibuster. Under Senate rules, 41 senators can tie up debate indefinitely, blocking a nomination.
Early yesterday, Senate minority leader Harry Reid and his deputy, Richard Durbin of Illinois, said they didn't think such a step was likely to be taken.
But later in the day, Kerry, the party's 2004 presidential nominee who has ambitions of making another run, and Kennedy, whose tough questioning of Alito had sparked widespread criticism, announced that they would mount a filibuster anyway.
Republicans quickly condemned the move.
''Continuing to threaten a filibuster, even after it is crystal clear that Democrats don't have the necessary votes to sustain their obstruction, is needless, strange, and at odds with many of their fellow Democrats," said Senator John Cornyn, Republican of Texas.
And Senate majority leader Bill Frist, Republican of Tennessee, quickly filed a petition to close the debate on Alito's nomination at 4:30 p.m. yesterday. Under Senate rules, Frist's move will force Democrats to vote on the filibuster on Monday afternoon. If Alito foes cannot muster 41 votes, the Senate will take a final vote on confirming Alito the following day.
Although many Democrats have said they will vote against confirming Alito, few have been enthusiastic about a filibuster because it is numerically unlikely to succeed. Democrats hold 44 seats in the Senate and would need 41 lawmakers to block an up-or-down vote on Alito's confirmation.
Already, at least three Democrats from conservative states -- Ben Nelson of Nebraska, Robert Byrd of West Virginia, and Tim Johnson of South Dakota -- have said they will support Alito. Several more have said they may vote against him but would not support a filibuster.
Still, liberal activists in the Democratic Party's base -- especially supporters of abortion rights -- have called upon Senate Democrats to go down fighting against Alito with a filibuster. By forcing a filibuster vote, Kennedy and Kerry are putting pressure on their colleagues, analysts said.
''It puts other Democrats in a very difficult and very awkward position, because they don't want to be outflanked," said Steve Rabinowitz, a Democratic strategist. ''It's tough for all good progressive Democrats who are trying to juggle politics and constituencies, and right and wrong, and who care about the Senate and its future, but it's especially difficult for the 2008 presidential candidates."
The move was the boldest political gesture in a year by Kerry, who has generally taken a lower profile since losing the 2004 presidential election to George W. Bush. Kerry may run again in 2008, where he could face off against several fellow Democratic senators for the nomination, including Hillary Rodham Clinton of New York, Joseph R. Biden Jr. of Delaware, and Evan Bayh of Indiana.
Political analysts yesterday said that the Kerry's decision to filibuster Alito could be the opening salvo in a 2008 primary campaign.
''It could be that Kerry wishes to position himself to the left of the others," said presidential scholar Stephen Hess. ''So that when he's standing up there debating Hillary Clinton . . . he can say, 'Look at what's happening on the Supreme Court, conservatives are turning the world upside down, and where were you when Alito's nomination came up?' "
Liberal activists cheered Kerry's and Kennedy's move. ''There is growing momentum against the Alito nomination," said Ralph G. Neas, president of the liberal group People for the American Way. ''The more debate, the more opposition there will be."
Several partisan analysts on both sides of the aisle were skeptical about whether a renegade filibuster bid made sense for Kerry's political future, however.
''If I were advising Kerry on this, I would tell him not to do it," said Democratic strategist Peter Fenn. ''It throws a wrench into the process that is already occurring. If he had announced it a month ago, that would be one thing. But in the eleventh hour to come out and say it, I think, will open him up to attack."
Whit Ayers, a Republican strategist, was dismissive. ''As a man with experience in lost causes, this makes sense," he said. ''Tilting at windmills has never been a calling card for a presidential contender."