Senate Democrats yesterday promised to subject John G. Roberts Jr. to an increased level of scrutiny in light of President Bush's decision to nominate the 50-year-old appeals court judge to replace the late William H. Rehnquist as chief justice.

But with conservatives and liberals alike saying that Roberts is on track to be confirmed, the focus was already shifting to what both sides believe will be the real battle: Bush's yet-to-be-named pick to replace retiring Justice Sandra Day O'Connor.

O'Connor's seat is critical because she often provided a swing vote on such controversial issues as affirmative action, abortion and prayer in public places. Roberts was initially chosen to replace her, but instead is now poised to succeed Rehnquist, a reliable conservative who died Saturday of thyroid cancer.

The rare opening of two seats on the nine-member court gives Bush the opportunity to dramatically move the balance of power on the court to the right.

The switch, which comes as the Bush administration struggles to deal with the aftermath of Hurricane Katrina, had both sides hurriedly recalibrating their strategies. Senate leaders agreed to postpone Roberts's confirmation hearings, which had been scheduled to start today, probably until next Monday.

Bush urged the Senate to quickly confirm Roberts in time for the Oct. 3 start of the new Supreme Court term, saying that the Senate was "well along in the process of considering Judge Roberts's qualifications."

But Senate Democrats said that the move to make Roberts the 17th chief justice of the United States required careful deliberation, particularly given Roberts's relatively short, two-year tenure as a federal appeals court judge.

"The stakes are higher and the Senate's advice and consent responsibility is even more important," said Senate Minority Leader Harry M. Reid (D-Nev.). "If confirmed to this lifetime job, John Roberts would become the leader of the third branch of the federal government and the most prominent judge in the nation."

Democrats on the Judiciary Committee plan to renew their calls for the White House to release memos and other documents from Roberts's 1989-1993 tenure as principal deputy solicitor general in the administration of President George H.W. Bush, his highest government posting. "Judge Roberts has a clear obligation to make his views known fully and completely," said Sen. Charles E. Schumer (D-N.Y).

Still, even liberal groups opposed to Roberts's nomination said yesterday that the shift is unlikely to alter his chances of being confirmed, given that Republicans are in firm control of the Senate. Some Republicans argued that it will even help his prospects, given that replacing Rehnquist with Roberts isn't likely to change the ideological center of the court.

Kate Michelman, a former president of NARAL Pro-Choice America, said that what senators must do now is highlight Roberts's record on civil rights, his skeptical writings about the legal underpinnings of abortion rights and other stances to "show people what it means to have his views on the court and lay the groundwork for the next nomination fight."

Democrats are already moving to link the two nominations. Sen. Edward M. Kennedy (D-Mass.) said that a review of memos that Roberts wrote while a young lawyer in the Reagan administration shows that Roberts sought to "weaken voting rights, roll back women's rights, and impede our progress toward a more equal nation."

Before the Senate acts on Roberts's new nomination, Kennedy said, the Senate has a right to know whom Bush intends to nominate for O'Connor's position. "The American people care deeply about the overall balance of their highest court."

With President Bush's approval ratings at an all-time low and his administration under fire over its handling of Hurricane Katrina, some conservatives are worried that Bush will forgo the chance to pick another conservative in favor of someone who will not provoke a fight.

Manuel Miranda, former legal counsel to Senate Majority Leader Bill Frist and the founder of a conservative group that follows judicial issues, said Bush must keep his campaign promise to nominate a justice in the tradition of conservative Justices Antonin Scalia and Clarence Thomas, or risk "sinking even lower in the polls."

"If anything is keeping him afloat right now, it's conservative support," Miranda said.

Meanwhile, Democrats on the Hill are feeling more emboldened by the president's weakened political standing, according to interviews with senior aides and Democratic strategists. Ron Klain, a former aide to President Bill Clinton, said Democrats may be more willing to fight a staunchly conservative second pick.

"As his poll numbers fall, his domestic problems accumulate, and as independent voters increasingly wonder if he is out of touch with what's going on in the country, then Democrats in conservative states have less to fear by crossing him," Klain said.

The question for Democrats is whether the American public, consumed with images of the devastation in the Gulf Coast states, will tune in to the nomination hearings.

Senators are treading carefully. Sen. Patrick J. Leahy (D-Vt.) said that the Senate looks forward to consulting Bush on a successor to O'Connor, a pick he stressed will "affect all of us, and the generations who follow us."

At the same time, he said, "these are not the only challenges facing the nation at this critical time. Most urgent at this moment is providing the necessary help to Americans still suffering in New Orleans and elsewhere along the Gulf Coast."

Sen. Edward M. Kennedy, right, said Roberts's memos show he sought to "impede our progress toward a more equal nation." Sen. Charles E. Schumer, left, renewed the call for more documents from Roberts's time as deputy solicitor general.