MYRTLE BEACH, S.C. -- Sen. John Kerry of Massachusetts, the 2004 Democratic presidential nominee, endorsed Sen. Barack Obama for the White House Thursday in a timely slap at Sen. Hillary Rodham Clinton as well as his own vice presidential running mate.

Interactive | Slideshow | Newsletter

Quoting a black American hero in endorsing the man who hopes to be the first black president, 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."

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," Kerry said.

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," said Kerry, who added that Obama, an opponent of the Iraq war, was "right about the war in Iraq from the beginning."

Kerry's announcement at a rally at the College of Charleston comes on the heels of two big union endorsements in Nevada.

Kerry, the Democrats' 2004 nominee who lost to George W. Bush, lost the South Carolina Democratic primary in 2004 to John Edwards, the former North Carolina senator who now is running third in the 2008 campaign behind Hillary Rodham Clinton and Obama.

Besides any potential help for Obama, Thursday's endorsement was a slap at Edwards, who was Kerry's running mate in the last election. The two had their differences during the campaign over strategy and spending. In post-mortem interviews, Edwards said he would have been more aggressive in challenging the unsubstantiated allegations of the Swift Boat Veterans for Truth, the Vietnam War veterans who questioned Kerry's military record.

Edwards responded to the endorsement with a statement released Thursday in Chapel Hill, N.C.: "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."

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

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

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

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 Bush.

As for Obama, Kerry gave the young Illinois state senator his first turn in the national spotlight when he chose him to deliver the keynote speech at the 2004 Democratic National Convention. Later that year, Obama won election as a U.S. senator.

Since announcing a year ago he would not make the run, 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.

Kerry should be able to provide some organizational muscle to Obama.

Since losing the 2004 race, 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.

On Wednesday, Nevada's 60,000 member Culinary Workers Union, Local 226, endorsed Obama. The 17,500-strong Nevada chapter of the Service Employees International Union endorsed him on Tuesday.