The embrace provides a dramatic rocket for Obama to ride into the frantic, nationwide campaigning ahead of the space of huge Super Tuesday primaries on Feb. 5, the biggest day for nominating contests in U.S. history.
The announcement stunned Senate colleagues, who had expected Kennedy to remain neutral until the increasingly vitriolic nominating contest with Sen. Hillary Clinton (D-N.Y.) settled out.
“This is the biggest Democratic endorsement Obama could possibly get short of Bill Clinton,” said a high-level Democrat.
The Clinton campaign launched a last-ditch effort over the few days to stop Kennedy's move, orchestrating a flood of phone calls to Kennedy from sources ranging from union chiefs to his Massachusetts constituents.
The former president also called Kennedy in a vain attempt to keep him out of the race, a source familiar with the conversation said.
During his two terms in the White House, President Clinton made repeated overtures to the Kennedy family. So the senator’s rejection of his wife is at least as embarrassing as her 28-point loss in the South Carolina primary on Saturday.
Kennedy is to appear with Obama at “Stand for Change” rally the campaign is holding at American University in Washington, D.C., at lunchtime on Monday, the sources said.
Word of Kennedy’s planned endorsement leaked the same day that his niece, Caroline Kennedy, endorsed Obama with a New York Times Op-Ed headlined, “A President Like My Father: Obama will inspire a new generation of Americans.”
The Kennedy endorsement is likely to give Obama a lift among Hispanic voters because of Kennedy's passionate advocacy of immigration legislation. The Obama campaign, which lags far behind Clinton among Hispanic voters in national polls, is likely to prominently display the endorsements by both Kennedys in Latino communities.
The disclosure also comes the same weekend that the House's highest-ranking Latino, California Rep. Xavier Becerra, also announced that he is backing Obama.
Obama was coy about the upcoming endorsement when ABC’s George Stephanopoulos asked him about it on “This Week.”
“I'll let Ted Kennedy speak for himself,” Obama said. “And nobody does it better. But obviously, any of the Democratic candidates would love to have Ted Kennedy's support. And we have certainly actively sought it.”
The Republican National Committee piled on, issuing a statement saying: “Senator Kennedy’s rejection of his friend and colleague is a stunning blow to Clinton’s campaign.”
Politico’s Ben Smith Carrie Budoff Brown, John F. Harris Gebe Martinez and Martin Kady II contributed to this report.