Some boom with portent, others drip with betrayal and still more sparkle with glamour, but presidential endorsements have rarely been so sought after — or pivotal to — campaigns.

As Super Tuesday reached its peak candidates often had to rely on dozens of high-profile supporters — so-called surrogates — who have been deployed coast-to-coast, across the airwaves and into cyberspace to deliver their message.

They can draw crowds in states where candidates are absent, reach sections of the electorate otherwise cut off from politics and, sometimes, launch negative attacks with which the candidates themselves do not wish to be associated.

Some of the most bruising “attack dog” comments in the Democratic race were made by Bill Clinton before he was forced back into his kennel amid some adverse voter reaction.

Chelsea Clinton has taken an extended leave of absence from her job at Avenue Capital Group to support her mother. After several weeks in which she played the part of a silent prop to Hillary Clinton, she has emerged as a significant and effective surrogate, particularly with young voters. She is 27 and some observers have suggested that the campaign may have “hard-wired” her to one day carry on her family's political dynasty.

Barack Obama has leant increasingly heavily on his pugnacious and witty wife, Michelle, and — in the past week — Edward Kennedy. The Clintons are understood to feel personally let down by Senator Kennedy's decision to endorse Mr Obama and then campaign so aggressively against them. In one telephone call Mr Clinton is understood to have listed the favours he had done for the Kennedy family when he was in the White House before pleading with him to at least stay neutral.

“Teddy has behaved very badly,” one senior Clinton source said. Another suggested that the Kennedys had always looked down on the former President from humble Southern roots who allegedly usurped some of their position in American politics.

On Mr Obama's side is Senator Kennedy and his son, the congressman Patrick Kennedy, together with Caroline Kennedy and Maria Shriver, the daughter and niece of President Kennedy, as well as Ethel Kennedy, the widow of Bobby Kennedy. Hillary Clinton is backed by Bobby Kennedy's three children, Robert Jr, Kathleen and Kerry.

Such divisions reflect how the Democratic race is splitting the party and even families. Arnold Schwarzenegger, California's Republican Governor, who has endorsed John McCain and is married to Ms Shriver, describes his bemusement at the different opinions emerging within the clan, adding: “I've been in the family for 30 years and I've never seen that.” Other prominent families at odds with themselves over the Democratic choice include Robert Rubin, the former Treasury Secretary, who is backing Mrs Clinton, while his son James is for Mr Obama.

The senior congressman Charles Rangel is supporting Mrs Clinton but his wife, Alma, is voting for Mr Obama. The Rev Jesse Jackson and one of his sons back Mr Obama while his wife and the other son support Mrs Clinton. Loretta and Linda Sanchez, the only pair of sisters in Congress, have also split between the two camps.

Celebrities have also been split by the Democratic race, often polarising along the same lines of age, race and gender that divide much of the electorate. Mrs Clinton gets support from Billie Jean King, Jack Nicholson, Barbra Streisand, Carly Simon, Martha Stewart and Elizabeth Taylor.

Mr Obama is backed by Oprah Winfrey, Toni Morrison, Scarlett Johansson, Matt Damon, George Clooney, Brad Pitt, Ben Affleck, Michael Jordan and Chris Rock.

He has also been boosted by a music video created in his honour by will.i.am of the band Black Eyed Peas that spread in viral fashion on the internet. Released on Friday, Yes I Can features his speeches spliced with many of his celebrity supporters. It had already received more than one million YouTube hits by Monday.

In the Republican race Mr McCain has reaped a huge harvest of endorsements in the past ten days since his wins in South Carolina and Florida convinced much of the party's Establishment that he was the likely nominee. Chief among these are Rudy Giuliani, the former front-runner for the nomination and former New York Mayor, as well as Mr Schwarzenegger in California. Mr Schwarzenegger, the former movie star and professional bodybuilder, has added his weight to an action hero theme within the campaign of Mr McCain, who is also backed by Sylvester Stallone, Harrison Ford and Tom Selleck.

Mike Huckabee has made much use of Chuck Norris, the martial arts expert and actor, as well as Ric Flair, a former professional wrestler. When Mr Norris alleged that Mr McCain was too old to be president, the Arizona senator promised to send Mr Stallone round to sort him out.

Mitt Romney, by contrast, has nobody with such punch. He is backed by Donny and Marie Osmond — fellow Mormons — and the singer Pat Boone, as well as the conservative commentators Ann Coulter, Sean Hannity, Lara Ingraham and Mark Levin.

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