Ms Winfrey (R) raised $3m for Mr Obama in September

Oprah's endorsement

Over 18,000 people attended the event in the state capital, Des Moines.

On 3 January, Iowa will hold the first caucus in the 2008 US presidential election, when activists will choose a candidate for their party's nomination.

Mr Obama's rival - Hillary Clinton - has also been campaigning in the state, accompanied by her mother and daughter.

'Compelled to speak'

The Oprah-Obama rally drew the biggest crowd of any campaign event in Iowa so far this year.

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"For the very first time in my life I feel compelled to stand up and speak out for the man who I believe has a new vision for America," Ms Winfrey told the crowd.

Ms Winfrey defended Mr Obama against charges that he lacks experience, saying "the amount of time you spend in Washington means nothing unless you are accountable for the judgment you made".

And she praised Mr Obama for "[standing] with clarity and conviction against this war in Iraq".

Iowans who had signed up to volunteer at least four hours of their time to the Obama campaign were given priority seats for the event.

Dozens of campaign workers were on hand to sign up fresh volunteers for the senator's electoral operation.

Ms Winfrey announced her endorsement of Mr Obama's candidacy in May, and hosted a fundraising event in September which raised $3 million (£1.5 million) for his campaign.

Key state

The race to win the Iowa Democratic caucus is very tight, with most polls showing Mr Obama, Mrs Clinton and another candidate - former Senator John Edwards - neck and neck.

Because it is the first state in the country to hold a caucus, presidential candidates are always keen to prove their electability by winning in Iowa.

Mrs Clinton is also campaigning hard in the state, and has appeared at an event with her mother Dorothy Rodham and her daughter Chelsea, vowing to bring "change across the generations".

In response to the Oprah-Obama rally, a Clinton staffer said "Senator Clinton is a big fan of Oprah's, and she thinks it is great for candidates to have surrogates campaign on their behalf".

Former President Bill Clinton has been supporting his wife's campaign, by appearing at events in another early primary state, South Carolina, while Hillary campaigns in Iowa.

Ms Winfrey is scheduled to attend a series of Obama campaign events in New Hampshire and South Carolina over the next few days.