CHARLESTON, S.C. — Senator Kerry of Massachusetts, the 2004 Democratic presidential nominee, endorsed Senator Obama for the White House today in a timely slap at Senator Clinton as well as his own vice presidential running mate.

Quoting a black American hero in endorsing the man who hopes to be the first black president, Mr. Kerry declared, “Martin Luther King Jr. said the time is always right to do what is right. And I’m here in South Carolina because this is the right time to share with you, to make sure that we know that I have the confidence … and that Barack Obama can be, will be, and should be the next president of the United States.”

Mr. Kerry delivered his endorsement in South Carolina at a time, two weeks before that state’s primary, when Mrs. Clinton is riding a wave of enthusiasm following her victory over Mr. Obama in the New Hampshire primary.

Mr. Kerry said there were other candidates in the race whom he also had worked with and respected.

“But I believe more than anyone else, Barack Obama can help our country turn the page and get America moving by uniting and ending the division we have faced,” Mr. Kerry said.

Mr. Kerry took a swipe at Obama critics who say the Illinois senator lacks the experience to be president.

“We are electing judgment and character, not years on this earth,” Mr. Kerry said, adding that Mr. Obama, an opponent of the Iraq war, was “right about the war in Iraq from the beginning.”

A former senator, John Edwards of North Carolina, the third contender in the Democratic presidential race, was Mr. Kerry’s vice presidential running mate in 2004. Despite their political alliance, the two men were not close personally and differed behind the scenes on campaign strategy in a race that President Bush won.

Mr. Edwards responded to word of the endorsement with a diplomatic statement: “Our country and our party are stronger because of John’s service, and I respect his decision. When we were running against each other and on the same ticket, John and I agreed on many issues.”

Mr. Kerry was Mr. Obama’s political benefactor once before, selecting the relatively unknown Illinois senatorial candidate to deliver the keynote address at the 2004 Democratic National Convention in Boston. It was Mr. Obama’s first turn in the national spotlight and helped launch him on a remarkable ascent that has made him one of two leading contenders for the party’s presidential nomination only four years later.

The Massachusetts senator lost the South Carolina Democratic primary in 2004 to Mr. Edwards.

Messrs. Kerry and Edwards had their differences during the 2004 campaign over strategy and spending, and Mr. Edwards has said he would have been more aggressive in challenging the unsubstantiated allegations of the Swift Boat Veterans for Truth questioning Mr. Kerry’s military record.

Mr. Kerry’s endorsement also was a jab at Mrs. Clinton, the New York Democrat who won the New Hampshire primary after a loss to Mr. Obama in the Iowa caucuses.

Mr. Kerry had withheld his endorsement, hoping to have an impact on the race and avoid the fate of a fellow Democrat, Al Gore, the 2000 nominee who endorsed Howard Dean in 2004 shortly before the former Vermont governor’s campaign imploded. Mr. Gore has made no endorsement so far this year.

While Mr. Kerry has been close to Mrs. Clinton’s husband, the former president, he was incensed in 2006 when she chided him after Mr. Kerry suggested that people who don’t go to school “get stuck in Iraq.” Aides said Mr. Kerry meant to jab at Mr. Bush and say “get us stuck in Iraq,” and that he didn’t appreciate Mrs. Clinton piling onto the criticism he was already getting for the remark.

The Republican National Committee pounced on the endorsement to brand Mr. Kerry and Mr. Obama “liberal soul mates.”

Mr. Kerry himself had considered running for president in 2008, but that plan fizzled with the botched remark. For many Democrats, his words revived bitter memories of his missteps in 2004, when he lost to Mr. Bush.

Since announcing a year ago he would not make the run, Mr. Kerry has prodded Democrats to take a stronger anti-war stance, pushing for troop withdrawal deadlines. In another area, he has backed environmental causes, writing a book with his wife on the issue.

Mr. Kerry should be able to provide some organizational and fundraising muscle to Mr. Obama.

Since losing the 2004 race, Mr. Kerry has kept a national network of supporters intact. He has an e-mail network of 3 million supporters, according to aides. He also has traveled extensively raising millions of dollars for Democratic candidates nationwide.