WASHINGTON – A debate began in the U.S. Senate yesterday over the all but certain confirmation of Judge John Roberts Jr. as America’s next chief justice. With 67 votes for the nominee already assured, senators from both parties decided to use their floor time advising President Bush on his next nominee rather than debate the merits of the current one, who is expected be confirmed Thursday.

Mr. Bush could announce his replacement for Justice O’Connor as soon as Friday. The president has offered few clues as to who his pick to replace Justice O’Connor will be, though he hinted yesterday that it will be a woman or a minority. Asked what role diversity will play in his pick, Mr. Bush said, “I am mindful that diversity is one of the strengths of the country.”

Speculation about a female nominee has centered on a federal Appeals Court judge for the Fifth Circuit, Priscilla Owen. Speculation about a Latino nominee has centered on Attorney General Alberto Gonzales. A black former deputy attorney general, Larry Thompson, is another minority whose name is often mentioned.

The president would likely avoid a fight from Democrats if he nominates Mr. Gonzales, who is not thought to be conservative on social issues. But his nomination could ignite a backlash among some Republican senators and from Mr. Bush’s base.

Judge Roberts is expected to pick up all 55 Republican votes and can expect at least 12 Democratic votes. A Democrat from Maryland, Senator Mikulski, yesterday added her vote to the list of the nominee’s opponents, which stood at 15 by late afternoon. Mrs. Mikulski said her decision was based on concerns about Judge Roberts’s potential impact on civil rights and abortion rights. “I have too many doubts about the direction that a Roberts court would take us, too many persistent, nagging doubts.” Mrs. Mikulski said.

A Democrat from Vermont who announced last week that he would vote for Judge Roberts, Senator Leahy, said yesterday that his affirmative vote was, in part, a signal to the White House about its next nominee. Citing party divisions over the response to Hurricane Katrina and the war in Iraq, Mr. Leahy urged Mr. Bush to choose a nominee who “unites us rather than divides us.” The Republican chairman of the Senate Judiciary Committee, Senator Specter, of Pennsylvania, issued a similar warning to Mr. Bush.

The executive director of the Committee for Justice, Sean Rushton, said that since Democrats will not be able to block a conservative nominee with their votes and could be rebuffed by the Republicans’ invocation of the “nuclear,” or constitutional option, if they attempt a filibuster, the president is in a position to nominate a conservative.

“If I’m the White House and I’m seriously considering going ahead with a conservative, I’m thinking, ‘Who do we figure out would not be filibustered?,'” Mr. Rushton said. “Call on the moderate Democrats and ask them, ‘Look, even if you would vote against Priscilla Owen, do you think she’d be filibuster-proof? As long as you get that, you’re okay. I mean, who cares, frankly, what Schumer and Leahy think?”