WASHINGTON - Republicans and Democrats alike are looking beyond John Roberts' virtually certain confirmation as the nation's 17th chief justice to President Bush's next nominee to the Supreme Court, expecting a quick announcement from the White House and a much tougher confirmation fight.

Three Democrats on the Senate Judiciary Committee sided with the panel's 10 Republicans on Thursday in a 13-5 vote to endorse Roberts, sending his nomination to the floor of the Senate for a confirmation vote no later than next Thursday.

With little suspense left in Roberts' climb to the Supreme Court, speculation focused on whether the support from three Democrats gives Bush any more or less leeway to nominate someone more conservative than Roberts to replace retiring Justice Sandra Day O'Connor.

"I think there were people in the White House who hoped the Democrats would all vote for him and show that Roberts was a high-quality nominee," said Stephen Wermiel, an American University law professor. "There were other people in the White House who hoped the Democrats would all vote against him, thinking that would help the president feel liberated to do whatever he wanted to do next."

Regardless of their motives, it was clear the Judiciary Committee members were eyeing Bush's next pick for the court.

"I think that some of the voting today was calculated to impact on the next nomination," said Sen. Arlen Specter, R-Pa., chairman of the committee. Some senators believed that backing Roberts puts them in a better position to oppose the next Bush nominee, while others thought that opposing him puts the president on notice that he had better put someone up who was acceptable to a broad spectrum of senators.

Norm Ornstein, a political analyst with the American Enterprise Institute, a conservative think tank in Washington, said anyone familiar with the Bush White House knows that a strong Democratic vote against Roberts would convince Bush that Democrats would reject anyone he'd nominate "so why bother nominating anyone but a fire-breathing conservative."

The White House, which has been keeping details of Bush's selection process secret, said only that the president planned to nominate a highly qualified candidate whom Americans can be proud to have on the court.

Some lawmakers and legal analysts claimed the three Democrats - Sens. Patrick Leahy, Russ Feingold and Herbert Kohl - likely supported Roberts so they would not be cast as obstructionists.

The idea of using the Roberts vote to position oneself for the next confirmation hearing was rejected by David Carle, a spokesman for Leahy, the ranking Democrat on the Judiciary Committee.

Lynn Becker, a spokesman for Kohl, D-Wis., said the senator considers every nominee based on his or her merits and casts his vote accordingly. And Feingold, also a Democrat from Wisconsin, hinted that while he voted for Roberts, he might not be willing to embrace a conservative federal appellate judge like Janice Rogers Brown, who has been mentioned as a possible replacement for O'Connor, the swing vote on issues including affirmative action, abortion, discrimination and death penalty cases.