Republicans and Democrats warned President Bush yesterday that his next pick for the Supreme Court will face much tougher scrutiny in the Senate, as Judiciary Chairman Arlen Specter lobbied the White House to delay the nomination until next year to defuse tension.

But the White House pushed ahead with plans to nominate Justice Sandra Day O'Connor's successor as early as the middle of next week from a shortlist that has been expanded beyond the field of candidates examined before the nomination of John G. Roberts Jr. to be chief justice and that includes several women and minorities, according to White House and Republican officials. First lady Laura Bush and a number of Republican senators are among those lobbying the president to nominate a woman or a minority, GOP officials said.

Whatever the nominee's sex or ethnicity, a Republican in close contact with the White House said the choice would be as conservative as Roberts.

After a morning briefing with Bush and top Senate leaders, Specter (R-Pa.) said he told the president he should postpone the announcement so senators have a better idea of how Roberts would influence the Supreme Court as chief justice over the next six months. Lawmakers say they expect Roberts to be confirmed easily next week. "I believe the next nomination is going to be a great deal more contentious than the Roberts nomination," Specter told reporters. "I say that because bubbling just below the surface was a lot of frustration in the hearing that we just concluded."

But the White House rejected the idea of delaying the next selection. Instead, a top White House aide said Bush plans to announce O'Connor's replacement next week, shortly after the Senate votes on Roberts's confirmation.

Roberts, who faces a vote in the Senate Judiciary Committee today, picked up an unexpected supporter yesterday: Sen. Patrick J. Leahy (Vt.), the top Democrat on the committee and a person liberal special interests groups were counting on to lead the Roberts opposition on the floor next week. In a Senate speech, Leahy heaped more criticism than praise on Roberts, saying Bush "has opted not to seek moderate candidates" for federal courts. He said Roberts "disserved himself by following the script that he had developed while serving in the Reagan administration." Opposing Roberts would be "easier and more popular" in his Democratic-leaning state, Leahy said, but he added that "I can only take him at his word that he does not have an ideological agenda."

Ralph G. Neas, president of the liberal People for the American Way, said Leahy "made all the arguments against the confirmation of Judge Roberts, and then made a decision that contradicted his own compelling reasoning. His decision was inexplicable, and deeply disappointing."

Sen. John F. Kerry (D-Mass.), who lost the presidential election to Bush in November, emerged as the first 2008 presidential contender to come out against Roberts, as did Sens. Edward M. Kennedy (D-Mass.) and Barbara Boxer (D-Calif.). Former senator John Edwards (D-N.C.) joined his former running mate in opposing the nomination.

It "may turn out that he will be an outstanding chief justice, but I cannot say with confidence that I know how he will approach constitutional questions of fundamental importance," Kerry said of Roberts in a floor speech. Because nominees duck so many questions, Kerry said, the confirmation process is "increasingly sterile" and "little more than an empty shell."

With little time to spare, Roberts responded yesterday to dozens of written questions that committee members submitted after last week's hearings. But if senators were hoping for last-minute insights into how the 50-year-old appellate judge would rule on controversial issues such as abortion, the death penalty, same-sex marriage and presidential war powers, they were disappointed. Roberts's written answers, like his testimony, steered clear of specifics or accurately summarized competing legal philosophies without stating his own.

Still, Roberts's confirmation is a done deal, according to vote counters on both sides. So senators and strategists are turning their attention to Bush's next pick -- someone both sides believe has the potential to change the ideological balance of the court for years to come. While Roberts is replacing a like-minded conservative, the late William H. Rehnquist, the next nominee will be replacing O'Connor, a swing vote on the court.

One GOP official, who would speak about conversations only on the condition of anonymity, said that some groups have urged administration officials to select a nominee who would excite the party's conservative base, on the theory that no one Bush nominates will attract significant Democratic support. But others have suggested trying to continue the momentum gained from the Roberts nomination with a conservative nominee who can generate more mainstream appeal.

White House officials are keeping the shortlist of possible nominees secret, but three Republicans close to the president said it includes men and women, whites and minorities. Among the names mentioned are Attorney General Alberto R. Gonzales; Edward Charles Prado and Priscilla R. Owen, both from the U.S. Court of Appeals for the 5th Circuit; Consuelo Maria Callahan, from the 9th Circuit; Larry D. Thompson, former deputy attorney general; Maura Corrigan, a member of the Michigan Supreme Court; Alice Batchelder of the 6th Circuit; Karen Williams and J. Michael Luttig, both from the 4th Circuit; Michael McConnell, from the 10th Circuit; and Samuel A. Alito Jr. of the 3rd Circuit.

In the morning meeting, Specter warned against nominating a controversial conservative such as Owen or District appellate judge Janice Rogers Brown, according to a Democrat briefed on the meeting. In the meeting, Leahy and Senate Minority Leader Harry M. Reid (D-Nev.) offered Bush several nominees they said would win bipartisan backing, including Sonia Sotomayor of the U.S. Court of Appeals for the 2nd Circuit, according to another person briefed on the meeting.

A Republican with close ties to the White House said "it is a little bit silly" to think the White House is zeroing in on women and minorities only. This official and others said many inside the White House believe whomever Bush picks will face such strong resistance that it has had the effect of liberating the president to go with the candidate he feels most comfortable with, even if that person is a white male.

Staff writers Dan Balz and Jo Becker and research editor Lucy Shackelford contributed to this report.