WASHINGTON – The chairman of the Senate Judiciary Committee yesterday said he will vote to confirm Judge John Roberts Jr. as chief justice of the United States, a conclusive sign the 50-year old Circuit Court judge and former Reagan administration lawyer will be voted out of committee this week and confirmed by the full Senate in the days ahead.

Some activist groups had hoped that the chairman, Senator Specter, would vote against the nominee because of his unwillingness to endorse Roe v. Wade, the 1973 decision that overturned state laws restricting abortion. But the Pennsylvania Republican said that Judge Roberts’s credentials, demeanor, and approach to the law impressed him enough to win his endorsement even without a commitment on the contentious abortion case.

Speaking to reporters after delivering his endorsement speech to a near empty Senate chamber, Mr. Specter said that while he is not sure how the nominee will rule on a challenge to Roe v. Wade, he is confident Judge Roberts’s stated commitment to judicial modesty and respect for precedent will not result in the reversal of contentious Supreme Court decisions such as Roe v. Wade that most Democrats and a few Republicans prize.

“I had some concerns about this nominee at first,” Mr. Specter said. “But many of my concerns were eliminated when I saw the caliber of judge that Judge Roberts is. I think the president disarmed his critics with this nominee.”

The judiciary committee will vote on Judge Roberts this Thursday and send the vote to the full Senate for debate. With Mr. Specter’s vote assured, Democrats are now weighing their own vote on the nominee. There was some speculation last week that Senator Kohl, a Democrat of Wisconsin, might vote for Judge Roberts. But Senate sources said yesterday that he is unlikely to vote alone. Another member possibly in favor of the nominee, Senator Feinstein, a Democrat of California, appeared to be leaning against a vote at the end of the hearing last week.

Democrats in the Senate face a quandary: whether to vote for someone whose position on a variety of issues remains unknown but whose competence and record are difficult to question. Republicans, particularly those closely associated with social conservatism, face a similar dilemma. One Republican source said struggles on both sides of the aisle indicate that Judge Roberts has succeeded already in steering the court away from its decades-long role as a final battleground where contentious issues are resolved.

“One of the bad consequences of the court becoming more of a political institution is that people tend to make the best guesses they can about a nominee,” a former colleague of Judge Roberts’s in the Reagan Administration, Peter Rusthoven, said. “What is clear is that a nominee, and in particular a nominee for chief justice should not play that game, and I think John did a remarkably good job of not playing that game.”

The chairwoman of Republicans for Choice, Ann Stone, this week reaffirmed her “cautiously optimistic” position on Judge Roberts. Citing conversations she has had with friends of the nominee, Ms. Stone said she does not think he will vote to overturn Roe v. Wade. She characterized him as not a first choice but far from the organization’s last choice either.

“Talking to Roberts’s longtime friends, people who have known him 20, 30 years,” Ms. Stone said, “People tell me he’s not going to do anything outrageous … Does that mean he’ll vote for every restriction that comes down the pike? Yeah, probably.”

Conservative groups, such as Concerned Women for America, have also rallied behind the nominee, exposing themselves to a potential backlash if Judge Roberts turns out to be more modest in the face of precedent than they would like. Mr. Rusthoven said the principle of judicial modesty Judge Roberts endorsed during last week’s hearing could make him an ally of either side in the debate over precedents.

“Is the right thing to do to say I realize this thing was wrongly decided and given the divisive impact it’s had and that it’s unworkable the thing to do is move away,” Judge Roberts said. “Or is it right to say, ‘Okay, it was wrong, but it’s been there for 30 years and it’s how people have come to view the law, and it’s not the role of the court to be disruptive.”

Some liberal advocacy groups who oppose Judge Roberts have already conceded defeat over the nomination and are gearing up for President Bush’s next pick. The founder and president of the anti-Roberts group Alliance for Justice, Nan Aaron, said she was not surprised by Mr. Specter’s decision to vote for Judge Roberts. She said she is now encouraging Democrats to vote against the nominee in the Senate as a sign to the president that his next pick should be a “consensus” pick.

“There’s no question at this point that he’s going to be confirmed,” Ms. Aaron said. “But I think a major reason Democrats should vote ‘No’ is to send a message to the White House that Bush should nominate a consensus nominee the next time. If these guys vote for him and then ask the base to come out and vote for them, it’s very damaging to the Democratic Party.”