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 once labeled Bill Clinton 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.