WASHINGTON — The battle over the next Supreme Court justice began in earnest yesterday, as Democrats refused to rule out a filibuster to block President Bush's nominee and senators from both parties urged special-interest groups to calm down.

With Justice Sandra Day O'Connor's announcement Friday that she will retire, the White House, lawmakers on Capitol Hill, and conservative and liberal groups began girding for an all-out political campaign to either affirm or reject the president's choice.

Sen. Joseph Biden, D-Del., a member of the Judiciary Committee, said on CBS' "Face the Nation" that he could envision circumstances that would justify a filibuster to block a nominee. For example, Janice Rogers Brown, who only recently joined the U.S. District Court of Appeals after a lengthy confirmation fight, could trigger that parliamentary maneuver, he said.

"I could assure you that would be a very, very, very difficult fight and she probably would be filibustered," Biden said, adding, "I have no intention of filibustering, but it depends on who the president sends."

On NBC's "Meet the Press," Sen. Patrick Leahy, D-Vt., the senior Democrat on the Judiciary Committee, said, "I would hope that we don't reach that point." However, he did not reject the possibility. Instead, he said Democrats would continue to urge Bush to choose someone who would unite the country.

"If you had somebody on the extreme right, just as if you had somebody on the extreme left, that's not going to unite the country and that's going to bring about a fight in the Congress," he said.

Similarly, Sen. Dianne Feinstein, D-Calif., the sole woman on the Judiciary panel, would not rule out blocking a nominee with a filibuster. "Certainly a filibuster is the last resort," Feinstein said on Fox News.

Republicans, including Sen. Orrin Hatch, R-Utah, said such a drastic tactic would likely backfire. Sixty votes are required to cut off a filibuster and proceed to the actual confirmation vote.

"I don't think the American people are going to put up with a filibuster against a Supreme Court judicial nominee," Hatch said on "Face the Nation." "I think if some of the Democrats advocate that, I think they're going to really hurt their party very badly."

A bipartisan agreement signed six weeks ago by seven Democrats and seven Republicans says a judicial nominee would be filibustered only in "extraordinary circumstances." Several of those senators said yesterday that a nominee's philosophical views could not amount to "extraordinary circumstances," and therefore a filibuster could be justified only on questions of personal ethics or character.

Senators from both parties criticized the special-interest groups gearing up to spend millions of dollars to either oppose or support the nominee. Conservative groups also have signaled that they would not go along with the president if he nominated Attorney General Alberto Gonzales.

"My reaction is that people ought to hold their fire and stand back and realize that it's President George W. Bush who was elected with the constitutional authority to make the designation," said Sen. Arlen Specter, R-Pa., chairman of the Judiciary Committee, who promised a professional and thorough round of hearings on the nominee.

Leahy noted that O'Connor herself would have endured a difficult confirmation fight if she were nominated today.

"I would urge that the groups on both the right and the left calm down a little," Leahy said. "Let's see who the nominee is and trust the Judiciary Committee to do a good hearing. You know, the irony is if Sandra Day O'Connor was being nominated today, the left would start complaining that 'We can't have her, she's a conservative Republican.' The right would complain, 'We can't have her, she's too much of a moderate and a consensus builder.' And yet every one of us will agree she's been a darn good justice."

For his part, Gonzales told reporters yesterday as he flew to Baghdad that he was not worried about what people were saying about him, The Associated Press reported.

"Many of the people speaking probably don't have all the information about prospective nominees," said Gonzales, a former member of the Texas Supreme Court appointed by Bush when the president was governor. "What's important is what the president of the United States thinks about me."

Material from The Washington Post is included in this report.