Barack Obama's campaign to become the Democratic candidate for US president received a major lift with the coveted backing of Senator Edward Kennedy.
The seal of approval from the patriarch of one of America's most prominent political dynasties could give fresh momentum to Mr Obama, analysts say.
The elder statesman made his endorsement public at a rally in Washington with the Illinois senator.
Mr Kennedy has reportedly been unhappy with the tone of the Clinton campaign.
The last surviving brother of assassinated President John F Kennedy, Senator Kennedy is now expected to campaign with his son, congressman Patrick Kennedy of Rhode Island, for the man who would be the first US black president.
In a prepared statement on Monday, Edward Kennedy praised Mr Obama as a "man with extraordinary gifts of leadership and character", adding: "I feel change in the air."
Nobel laureate African-American author Toni Morrison, who once dubbed Bill Clinton the "first black president", also endorsed Mr Obama on Monday.
Mr Obama, 46, decisively beat his rival, Senator Hillary Clinton, in South Carolina's party nomination battle on Saturday, after Mrs Clinton won in New Hampshire and Nevada.
Analysts say Mr Kennedy's backing appears linked to criticism that ex-President Clinton's campaigning on behalf of his wife in South Carolina may have been overly aggressive.
Mr Kennedy and Mr Clinton had a heated telephone exchange earlier this month, according to Monday's New York Times.
The Massachusetts politician reportedly accused Mr Clinton of injecting race into the campaign and complained over what he considered to be misleading statements by Mr Clinton about Mr Obama.
The BBC's Vincent Dowd in Washington says Mr Obama has been hoping for some time for the endorsement of the 75- year-old but Mrs Clinton's camp has been lobbying for Mr Kennedy to at least remain neutral.
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After news of Mr Kennedy's backing for Mr Obama broke, the Clinton camp distributed a statement from Kathleen Kennedy Townsend, the daughter of Robert F Kennedy.
The former Maryland lieutenant governor pledged her support to Mrs Clinton.
Democratic sources told Monday's Washington Post that Edward Kennedy had been seriously considering backing Mr Obama since he won Iowa's caucuses on 3 January.
Mr Obama beat Mrs Clinton in the weekend's South Carolina primary election by 55% to 27%, with John Edwards third on 18%.
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In the tally of primary and caucus contests so far, the two Democratic front-runners have each won a brace - Mr Obama at Iowa and South Carolina, while Mrs Clinton triumphed in New Hampshire and Nevada.
Analysts say the contest could extend beyond Super Tuesday next week, when two dozen states hold primary elections.