Mr. Kennedy called Mr. Clinton Sunday to tell him of his decision.
The endorsement, which followed a public appeal on Mr. Obama’s behalf by Caroline Kennedy, the daughter of President John F. Kennedy, was a blow to the Clinton campaign and pits leading members of the nation’s most prominent Democratic families against one another.
Mr. Kennedy, a major figure in party politics for more than 40 years, intends to campaign aggressively for Mr. Obama, beginning with an appearance and rally with him in Washington on Monday. He will be introduced by Ms. Kennedy.
Mr. Kennedy then heads west with Mr. Obama, followed by appearances in the Northeast. Strategists see him bolstering Mr. Obama’s credibility and helping him firm up support from unions and Hispanics, as well as the party base.
The endorsement appears to support assertions that Mr. Clinton’s campaigning on behalf of his wife in South Carolina has in some ways hurt her candidacy.
Campaign officials, without acknowledging any faults on Mr. Clinton’s part, have said they will change tactics and try to shift Mr. Clinton back into the role he played before her loss in the Iowa caucuses, emphasizing her record and experience.
Mr. Kennedy, of Massachusetts, has worked closely with Mrs. Clinton, of New York, on health care and other legislation and has had a friendly relationship with both Clintons, but associates said he was intrigued by Mr. Obama’s seeming ability to inspire political interest in a new generation. For his part, Mr. Obama actively courted Mr. Kennedy for several years, seeking him out for Senate advice and guidance before making the decision to enter the presidential race.
Mr. Kennedy had been seriously considering an endorsement for weeks — a break with his traditional practice of staying clear of primaries.
He remained uncertain of his decision as late as the middle of last week. But, according to allies, when he learned that his niece’s endorsement would appear as an Op-Ed piece in The New York Times on Sunday, he decided to bolster that with his own public embrace of the campaign at a joint rally at American University in Washington on Monday, giving Mr. Obama, of Illinois a potentially powerful one-two Kennedy punch.
As Mr. Obama flew here on Sunday, he smiled when asked about his new wave of support from the Kennedy family.
“For somebody who, I think, has been such an important part of our national imagination and who generally shies away from involvement in day-to-day politics to step out like that is something that I’m very grateful for,” Mr. Obama said of Caroline Kennedy’s support. Ms. Kennedy declined requests on Sunday to discuss her endorsement.
Trying to dilute the impact of the twin endorsements by the brother and daughter of the late president, the Clinton campaign on Sunday issued a statement of support from Kathleen Kennedy Townsend, a former lieutenant governor in Maryland and a daughter of Robert F. Kennedy.
“I respect Caroline and Teddy’s decision, but I have made a different choice,” Ms. Townsend said in her statement, adding: “At this moment when so much is at stake at home and overseas, I urge our fellow Americans to support Hillary Clinton. That is why my brother Bobby, my sister Kerry, and I are supporting Hillary Clinton.”
But two years ago, Ms. Townsend’s mother, Ethel Kennedy, referred to Mr. Obama in an interview as “our next president” and likened him to her late husband.
The Kennedy endorsement grants Mr. Obama, who has been framed by the Clintons as being short on experience, the approval of one of the Senate’s senior members.
Before the Iowa caucuses, Mr. Kennedy had planned to stay out of the race, largely because he had so many friends in the contest, chiefly Senator Christopher J. Dodd of Connecticut. He also said he was waiting for one of the candidates to spark a movement.
“I want to see who out there is going to be able to inspire not only our party, but others, because I think we’re going to need the inspiration in order to bring a change in American foreign policy and domestic policy,” Mr. Kennedy said last year on ABC News’s “This Week.”
After Mr. Obama won the Iowa caucuses, associates to both men said, Mr. Kennedy concluded that Mr. Obama had transcended racial lines and the historical divisions the Kennedy family had worked to tear down. Mr. Kennedy was also impressed at how Mr. Obama was not defined as a black candidate, but seen as a transformational figure.
It was then, associates said, that Mr. Kennedy began talking with his children, nieces and nephews, including Caroline Kennedy, who had reached her own judgment some time ago independently of her uncle. They then agreed last week to move ahead with their endorsements, coordinating their decision before the Feb. 5 contests.
Mr. Kennedy has a long history of working with the former president and Mrs. Clinton on health, education and other social issues and, according to his associates, has a good relationship with both. While the Clintons were in the White House, the families socialized and sailed off Cape Cod.
Mr. Obama courted Mr. Kennedy as well, using late-night sessions in the Senate to get some tutoring about the intricacies of the institution. Conversations about the White House began more than a year ago, with Mr. Obama paying Mr. Kennedy a visit to seek his thoughts about whether he should run for president. Mr. Kennedy told him that he should because such opportunities rarely come along.
On the night of Mr. Obama’s national political debut at the Democratic National Convention in 2004, he was preceded on stage by Mr. Kennedy, a symbolic bookend of the party’s dean and its new generation.
A year later, near the end of Mr. Obama’s first year in the Senate, Ethel Kennedy asked him to speak at a ceremony for her husband’s 80th birthday. At the time, she referred to Mr. Obama as “our next president.”
“I think he feels it. He feels it just like Bobby did,” Mrs. Kennedy said in an interview that day, comparing her late husband’s quest for social justice to Mr. Obama’s. “He has the passion in his heart. He’s not selling you. It’s just him.”