Maria Shriver, niece of John F. Kennedy and wife of Governor Arnold Schwarzenegger of California, delivered a media coup for the Obama campaign yesterday with a surprise endorsement of the Democratic candidate at a star-studded LA rally.

Casually dressed and make-up free after attending a daughter’s equestrian competition, California’s first lady joined Oprah Winfrey and Caroline Kennedy on stage at UCLA to declare her support for Mr Obama, who she said could unite America across racial, ethnic and other divisions.

"He's not about himself," she told a wildly cheering crowd at the university’s Pauley Pavilion. "He's about the power of us and what we can do if we come together. . . . He is about empowering women, African Americans, Latinos, older people, young people. He's about empowering all of us."

"I thought, if Barack Obama was a state, he'd be California," Ms Shriver said to increasingly clamorous applause. "Diverse. Open. Smart. Independent. Bucks tradition. Innovative. Inspirational. Dreamer. Leader."

Coming just days after her husband’s endorsement of John McCain, the Republican frontrunner, Ms Shriver’s appearance was an apparently impromptu one. Though a fiercely loyal Democrat, she was reportedly wavering up to the last minute about going public, and, even as she waited backstage, Obama aides were unsure she would come through.

But ultimately she overcame her qualms to deliver a PR coup for the Obama camp – and a stinging blow for the Clinton campaign.

Embracing Winfrey, Ms Kennedy – her first cousin – and Michelle Obama in front of the cameras, the image of three of the most influential female Democrats united in their support for the young Illinois senator dominated US news coverage just two days ahead of the crucial Super Tuesday nominating contests.

Ms Shriver’s break with her husband – to align herself with much of the Kennedy clam – was not unusual, as the pair being famous political opposites.

But like many, she said, she felt pulled in several directions.

“I wasn’t on the schedule,” she said, “and I thought to myself when I woke up this morning, I thought, there’s no other place I should be than right here.”

"Every single person can come to you and tell you, 'You should vote for this person' or 'You should vote for that person.' `It's not right to speak up. It's not right to speak out,' " Ms Shriver explained to an enraptured audience of thousands.

"But this is a moment to have a conversation with yourself, not anyone else. Have a conversation with your own heart. And ask yourself, 'What kind of America do I believe in?' "

Like the Democratic Party itself, the Kennedy family too is divided in their support for the two top presidential contenders. Senator Ted Kennedy, his son Patrick and niece Caroline, the daughter of President Kennedy had already spoken out on behalf of Obama, while on Saturday, Ethel Kennedy, widow of Robert, joined them with an endorsement.

Meanwhile Hillary Clinton has picked up the support of three of Robert and Ethel's children: Kathleen Kennedy Townsend, Kerry Kennedy and Robert F. Kennedy Jr.

The sheer drama of Ms Shriver's endorsement – at the end of a rally which had already seen its fair share of celebrities, including Stevie Wonder – guaranteed Mr Obama extensive TV coverage in California in the final 24 hours of the campaign.

It knocked Mr Clinton, who had spent Sunday morning visiting African-American churches in the LA area, out of the top news slot, a key advantage in a state where Mr Obama has eroded a monumental Clinton lead to challenge the former first lady for the state's slew of primary votes.

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