The upcoming battle over a successor to Supreme Court Justice Sandra Day O'Connor threatens to plunge the Senate into another bitter confrontation over filibusters and the "nuclear option," with Democrats already threatening to use any means possible to thwart President Bush if he nominates someone they regard as too conservative.

The roster of those threatening a filibuster includes liberal and moderate Democrats, supporters and opponents of John G. Roberts Jr., Democratic National Committee Chairman Howard Dean, and at least one of the seven Democratic senators who were part of the bipartisan "Gang of 14."

Democrats have splintered almost evenly over Roberts's nomination as chief justice, leading to frustration among party activists who think their elected leaders did not put up a serious fight against him. Pollsters have told party leaders that a show of opposition against Bush's next nominee could be crucial to restoring enthusiasm among the rank and file on the left.

In an interview, Dean said Democratic unity is essential in the upcoming battle and that the party "absolutely" should be prepared to filibuster -- holding unlimited debate and preventing an up-or-down vote -- Bush's next high court nominee, if he taps someone they find unacceptably ideological. He cited appellate court judges Priscilla R. Owen and Janice Rogers Brown as two who would be likely to trigger such opposition.

"Those people are clearly not qualified to sit on the Supreme Court, and we're going to do everything we can to make sure they don't," he said. "If we lose, better to go down fighting and standing for what we believe in, because we will not win an election if the public doesn't think we'll stand up for what we believe in."

The possibility of a filibuster comes only a few months after an agreement that supposedly eliminated such threats. The Gang of 14 agreement barred filibusters against judicial nominees except under "extraordinary circumstances." The compromise also blocked Republican threats to change Senate rules to bar the use of filibusters to block judicial nominations, a step considered so drastic it became known as the "nuclear option."

Owen and Brown were cleared for confirmation to the appellate courts as part of that agreement, and Republicans said then that Democratic acquiescence in their confirmation meant the opposition party could not use ideology to bar future Bush nominees. But Democrats rejected that interpretation and said this week that Owen, Brown and several others believed to be under consideration by the president face a likely filibuster if nominated to the high court.

A spokesman for Senate Minority Leader Harry M. Reid (D-Nev.) declined to issue a blanket filibuster threat but joined Dean in saying that a nominee judged more conservative than Roberts will face vigorous opposition because the successor to O'Connor -- the key swing vote on many issues -- could shift the ideological balance of the court.

"A nominee more extreme than Judge Roberts would be unacceptable to the Democratic caucus," said Reid spokesman Jim Manley, who added later: "You could expect a major fight on the Senate floor."

Sen. Ken Salazar (D-Colo.), a member of the Gang of 14 who plans to vote for Roberts, said a filibuster will be warranted if "the president appoints someone who brings a right-wing ideology and is going to use the court to advance their views."

In an interview yesterday, Salazar said: "From my personal point of view, anyone who is going to be an ideologue, who is going to have right-wing views, falls within that category of extraordinary circumstances." He said that although he would attempt to defeat such a nominee by enlisting opposition among moderate Republicans, "a filibuster has to remain a procedural possibility."

Sen. Mark Pryor (D-Ark.), also a member of the Gang of 14, said he hopes White House consultation with senators would persuade the president to select a consensus nominee. "If he sends over someone who looks like a conservative ideologue, who's going to be an activist on the court, that could be very problematic," he said.

Sen. Ben Nelson (D-Neb.), who was instrumental in forging the May agreement, said: "I'm not anxious to speculate what might trigger an extraordinary circumstance. If there is one, I'll know it when I see it. It's just very difficult to try to do that without a particular nominee in place."

The Senate is scheduled to vote on Thursday on Roberts's nomination to become the 17th chief justice of the United States, and White House officials have said Bush will move quickly after the vote to announce his nominee to succeed O'Connor. That could mean an announcement by Friday or possibly early next week.

If Democrats threaten a filibuster, Republicans are likely to respond by bringing back the nuclear option. Sens. Lindsey O. Graham (S.C.) and Mike DeWine (Ohio) have said that if any Democrats in the Gang of 14 join a filibuster, they will support invoking the nuclear option, providing enough votes to assure passage.

Democrats think Bush is too weak politically to take on a difficult fight over the court. Some also argue that Republicans would lose politically if they change the rules to force through a nominee.

The speculation about a filibuster comes even before the coalition of liberal groups leading the opposition to Bush's court picks has settled on a strategy to press on Democratic senators in the next nomination fight. "No one is talking about filibuster at this point," said Nan Aron, president of the Alliance for Justice. "It's much too early."

Dean said a straight party-line vote would show Democratic unity but would not be sufficient to block a nominee. "That's not a fight to the death," he said. "A fight to the death is a filibuster, which is the only way we can reject an unqualified nominee -- because the Republicans don't seem to have any qualms about putting unqualified people in all manner of positions all over the government."

Staff writers Michael A. Fletcher and Jim VandeHei contributed to this report.

Howard Dean of the Democratic National Committee said his party might filibuster a Supreme Court pick considered too conservative.