WASHINGTON – The top-ranking Democrat on the Senate’s judiciary committee said yesterday that he will vote to confirm President Bush’s nominee to be chief justice, Judge John Roberts Jr., disappointing a number of special interest groups that were hoping for a united Democratic front against Judge Roberts and surprising some of his Republican backers who were disapprovingly but eagerly expecting the same.

The announcement by Senator Leahy, a Democrat of Vermont, came shortly after a meeting with President Bush on potential candidates for another open seat on the court and one day before the judiciary committee is scheduled to vote on Judge Roberts. It came one day after the Senate’s Democratic minority leader, Senator Reid, of Nevada, said he will vote against Judge Roberts when his confirmation comes before the full Senate next week.

In departing from his party’s leader in the Senate, Mr. Leahy outlined a number of areas in which he was dissatisfied with Judge Roberts’s answers during his confirmation hearing last week. In particular, he cited the nominee’s unwillingness to distance himself from the policies of the Reagan Administration and for taking a “narrow” view of a judicial ethics rule that limits what nominees say about cases that might come before them in the future.

In the end, though, Mr. Leahy said he decided to vote for Judge Roberts after making the judgment that he is not an ideologue bent on shaping the law to reflect a personal agenda. Judge Roberts submitted answers yesterday afternoon to a number of new questions from committee members, sticking closely to the script he used during last week’s hearing.

“Judge Roberts is a man of integrity,” Mr. Leahy said. “I can only take him at his word that he does not have an ideological agenda. For me, a vote to confirm requires faith that the words he spoke to us have meaning.”

Mr. Leahy’s announcement sent shock waves through an activist community that has been gearing up for years to oppose conservative judicial nominees to the court. Soon after Mr. Reid announced his opposition to Judge Roberts, the group People for the American Way touted that “Opposition to Judge Roberts Gains Momentum” on its Web site. But after Mr. Leahy’s announcement, the group’s president issued a statement decrying the split in party ranks.

Frustration on the part of interest groups first surfaced last week, when the president of the Alliance for Justice, Nan Aron, chided Democrats for going easy on Judge Roberts during early questioning. The assault continued over the weekend, when, according to the political newspaper Roll Call, the founder of People for the American Way, Norman Lear, upbraided two of Mr. Leahy’s Democratic colleagues on the judiciary committee, Senator Schumer, of New York, and Senator Durbin, of Illinois, at a group meeting in California.

Just moments after Mr. Leahy’s announcement, Senator Kennedy, a Democrat of Massachusetts, announced that he will vote against Judge Roberts. Another Massachusetts Democrat, Senator Kerry, also declared his intention to vote against the nominee. Mr. Kerry lost a presidential bid against Mr. Bush last year and is said to be eyeing another run in 2008. His opposition was viewed by some as political positioning ahead of that race.

Other Democrats are making political calculations of their own. Two Democrats from states that voted for Mr. Bush last year, Senator Nelson of Nebraska and Senator Conrad of North Dakota, have indicated they will vote for Judge Roberts. Senator Clinton, a Democrat of New York who has been at pains to associate herself with moderate positions ahead of an anticipated 2008 run, has been silent on the nomination.

Westchester District Attorney Jeanine Pirro, who is seeking the Republican nomination to challenge Senator Clinton, has said that she supports Judge Roberts’s nomination. While Ms. Pirro would like to maintain Roe v. Wade, she believes that Judge Roberts has proven himself as a fair jurist during the nomination hearings.

New York’s other Democratic senator, Senator Schumer, is under pressure to vote against Judge Roberts because of his position as head of the Democratic Senatorial Campaign Committee.

Some have said that Mr. Schumer, who voted against Judge Roberts’s nomination to the U.S. District Court for the D.C. Circuit two years ago, may vote for him this time in order to gain credibility in voting against the president’s next nominee. Justice O’Connor said in July that she would like to leave the court to take care of her husband, who suffers from Alzheimer’s disease. Senator Specter, a Republican of Pennsylvania who chairs the judiciary committee, urged Mr. Bush yesterday to wait until the end of the year before naming a replacement for Justice O’Connor.

The growing split among Democrats on the nomination of Judge Roberts has perplexed some supporters of the president’s nominees, who said the division weakens the party ahead of another battle. A former deputy assistant attorney general, Shannen Coffin, said Democrats are arguing from a position of weakness not because of their lack of unity over Judge Roberts, but because of their minority status in the Senate. Still, he said, the division will not help their cause.

A former dean of the Boston University School of Law who is co-chairman of the pro-Roberts group the Committee for Justice, Ronald Cass, said Mr. Leahy’s speech was consistent with previous statements he has made about the Senate’s role in “double-checking” the president’s judicial nominee. Mr. Leahy voted for Justice O’Connor and Justice Antonin Scalia. He voted against Justice Clarence Thomas.