The Kennedy charisma and connections will come as a big boost to the Obama campaign before the 22-state Feb. 5 showdown for the Democratic nomination. The Clintons and their allies had pressed Kennedy for weeks to remain neutral in the Democratic race, but according to the New York Times, the latter has become increasingly disenchanted with the tone of the Clinton campaign.
According to the paper, Kennedy and Bill Clinton had a heated telephone exchange earlier this month over what Kennedy considered misleading statements by Clinton about Obama, as well as his injection of race into the campaign.
Kennedy called Clinton Sunday to tell him of his decision.
The endorsement, which followed a public appeal on 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.
Kennedy, a major figure in party politics for more than 40 years, intends to campaign aggressively for Obama, beginning with an appearance and rally with him in Washington on Monday.
Kennedy then heads west with Obama, followed by appearances in the Northeast. Strategists see him bolstering 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 Clinton's campaigning on behalf of his wife in South Carolina has in some ways hurt her candidacy.
Kennedy has worked closely with Senator Hillary 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 Obama's seeming ability to inspire political interest in a new generation.
For his part, Obama actively courted Kennedy for several years, seeking him out for Senate advice and guidance before making the decision to enter the presidential race.
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.