MoveOn.org claims to have 3.2 million members, and said it would start a campaign immediately to persuade them to support the senator from Illinois.

Mr Obama is vying with New York Senator Hillary Clinton to be the Democratic Party's candidate for president.

Election contests for both parties are held in more than 20 states on "Super Tuesday" next week.

Mr Obama has won more delegates than Mrs Clinton from the Democrats' contests so far, but polls suggest Mrs Clinton will have the edge in the states that vote on Super Tuesday.

Our members' endorsement of Senator Obama is a clear call for a new America at this critical moment in history

Eli Pariser

MoveOn.org executive director

For the Republicans, Senator John McCain has emerged as the front-runner after winning the primary election in Florida. His main rival is former Massachusetts Governor Mitt Romney.

Super Tuesday could be decisive for the Republicans, as in many states the winner of the contest takes all of the delegates on offer.

In the Democratic race, however, states award their delegates according to the proportion of the vote won - meaning the tight battle could continue for weeks.

'Desperate for change'

MoveOn said 70% of its members had backed Mr Obama over 30% for Mrs Clinton.

"Our members' endorsement of Senator Obama is a clear call for a new America at this critical moment in history," said MoveOn executive director Eli Pariser.

He says MoveOn's goal is to bring "progressive values" to Washington,

"Seven years of the disastrous policies of the Bush administration have left the country desperate for change."

Mr Pariser said America needed its next president "to end the war in Iraq, provide health care to every American, deal with our climate crisis, and restore America's standing in the world".

In another development, a mobile advertising company is to begin a campaign to urge New York Mayor Michael Bloomberg to run for president as an independent.

The billionaire Mr Bloomberg has insisted he does not intend to run, but a van set to drive the streets of New York next week proclaims him as: "The only presidential candidate that can't be bought."

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