When she begins a typical show, seldom does Oprah Winfrey question the power of her brand or wonder aloud about the influence that accompanies her golden seal of approval.

But when Ms Winfrey strode onto a stage here Saturday, imploring Iowa voters to support the presidential candidacy of Senator Barack Obama, she acknowledged not knowing whether her endorsement would matter.

And as she waded into American politics deeper than ever before, she declared, “It feels like I am out of my pew.”

Still, Ms Winfrey’s arrival on the campaign trail and her irrefutable appeal was threatening enough for Senator Hillary Rodham Clinton of New York to invite a famous guest of her own, daughter Chelsea Clinton, to appear with her on the political circuit in Iowa.

It was, perhaps, the best way Ms Clinton could be assured attention on a political day devoted to Ms Winfrey, who drew more than 15,000 people here and a second audience of about 7,000 in Cedar Rapids. In a state awash in presidential politics, with a competitive fight on both sides of the ticket, it was the largest spectacle of the campaign cycle.

The intersection of politics and celebrity began the moment Mr Obama’s wife, Michelle Obama, declared, “It is my honour to introduce to you the first lady of television, Oprah Winfrey.”  Ms Winfrey delivered a testimonial for Mr. Obama arguing the nation was at a critical moment in its history and required a candidate who could heal divisions and chart a new direction. She dismissed suggestions that voters should choose a candidate with the most Washington experience.

“If we continue to do the same things over and over again, I believe we get the same results,” Ms Winfrey said.

In his quest to win the party’s nomination and to become the first black president, Mr Obama is working to expand his reach beyond the traditional set of Democratic primary voters. The campaign hoped to use Ms Winfrey’s visit to Iowa on Saturday, followed by New Hampshire and South Carolina on Sunday, to expose his candidacy to a new audience of prospective supporters.

“There are those who say that Barack Obama should wait his turn. There are those who say that he should take a gradual approach to presidential leadership. But none of us is God,” Ms Winfrey said. “We don’t know what the future holds, so we must respond to the pressures and the fortunes of history when the moment strikes. And Iowa, I believe that moment is now.”

When she hailed his opposition to the Iraq war, “long before it was the popular thing to do,” the crowd responded with a roar of approval.