"I have marveled at his grit and grace," he said of the man a full generation younger than he is.
Kennedy's endorsement was ardently sought by all three of the remaining presidential contenders, and he delivered it at a pivotal time in the race. A liberal lion in his fifth decade in the Senate, the Massachusetts senator is in a position to help Obama court Hispanic voters as well as rank-and-file members of labor unions, two key elements of the Democratic Party.
He is expected to campaign actively for Obama in the days before a string of delegate-rich primaries and caucuses across 24 states on Feb. 5, beginning later this week in Arizona, New Mexico and California.
The senator made his comments at a crowded campaign rally that took on the appearances of a Kennedy family embrace of Obama, who sat smiling as he heard their praise.
He was introduced by Caroline Kennedy, daughter of the late president, who said Obama "offers that same sense of hope and inspiration" as did her father. Rep. Patrick Kennedy also endorsed Obama from the stage before a boisterous crowd at American University.
"Today isn't just about politics for me. It's personal," Obama, 46, said when it came time for him to speak. "I was too young to remember John Kennedy and I was just a child when Robert Kennedy ran for president. But in the stories I heard growing up, I saw how my grandparents and mother spoke about them, and about that period in our nation's life - as a time of great hope and achievement."
In his remarks, Sen. Kennedy sought one by one to rebut many of the arguments leveled by Obama's critics.
"From the beginning, he opposed the war in Iraq. And let no one deny that truth," he said, an obvious reference to former President Clinton's statement that Obama's early anti-war stance was a "fairy tale."
"With Barack Obama, we will turn the page on the old politics of misrepresentation and distortion.
"With Barack Obama we will close the book on the old politics of race against race, gender against gender, ethnic group against ethnic group, and straight against gay," Kennedy said.
The Massachusetts senator had remained on the sideline of the presidential campaign for months, saying he was friends with Obama, Clinton and former North Carolina Sen. John Edwards, as well as several Senate colleagues who are no longer in the race.
Lately, according to several associates, Kennedy became angered with what he viewed as racially divisive comments by Bill Clinton. Nearly two weeks ago, he played a personal role in arranging a brief truce between the Clintons and Obama on the issue.
Kennedy refers only sparingly to his assassinated brothers, John and Robert, in his public remarks, and his endorsement of Obama was cast in terms that aides said was unusually personal.
"There was another time, when another young candidate was running for president and challenging America to cross a new frontier. He faced criticism from the preceding Democratic president, who was widely respected in the party," Kennedy said, referring to Harry S. Truman.
"And John Kennedy replied, 'The world is changing. The old ways will not do. ... It is time for a new generation of leadership.
"So it is with Barack Obama," he added.
Kennedy began his remarks by paying tribute to Sen. Clinton's advocacy for issues such as health care and women's rights. "Whoever is our nominee will have my enthusiastic support," he said.
But he quickly pivoted to a strong endorsement of Obama, whom he said "has extraordinary gifts of leadership and character, matched to the extraordinary demands of this moment in history."
"I believe that a wave of change is moving across America," Kennedy said.
Also Monday, Obama picked up the endorsement of author Toni Morrison, who read from her work at Bill Clinton's first inauguration and once labeled him the "first black president." Morrison said she has admired Hillary Clinton for years because of her knowledge and mastery of politics, but cited Obama's "creative imagination which coupled with brilliance equals wisdom."
Morrison said her endorsement had little to do with Obama's race - he is the son of a black father from Kenya and a white mother from Kansas - but rather his personal gifts.