DES MOINES, Iowa - Oprah Winfrey put her star power behind Sen. Barack Obama in person yesterday, telling a rapt audience of thousands in Iowa that she was joining the fight for the White House because she is "so tired" of the status quo in Washington.

"You know I've never done this before, and it feels like I'm out of my pew," Winfrey told the crowd. "I'm nervous."

Without mentioning Obama's chief rival, Sen. Hillary Rodham Clinton, by name, Winfrey made a vigorous case against her, saying she was concerned that "if we continued to do the same things over and over and over again, I know that you get the same results."

The dramatic appearance at a packed arena overlooking the state capitol by the popular talk-show host - her first on behalf of a presidential candidate - underscored the high stakes in the nation's first caucuses, which will be held Jan. 3. Running neck and neck in the polls here, Clinton and Obama are campaigning furiously, with an emphasis on winning over female voters.

Clinging to her role as the front-runner, Clinton scrambled to match the moment with her own advocates, bringing in her mother and her daughter, Chelsea, to campaign with her for the first time. Clinton struck a low-key note, all but conceding Obama's high-wattage events would dominate the weekend news, and continued her efforts to get Iowans to turn out for her on caucus day.

Obama described Winfrey as "someone who moves an entire nation" and acknowledged that he was not the main attraction of the day. Winfrey shifted in her seat nervously as Obama piled on praise. "This is a wonderful person. We love her. I am grateful for her being here."

Polls show the top three Democrats - Obama, Clinton and former Sen. John Edwards - bunched closely at the front of the pack in Iowa. A central pivot point in the race, especially between Clinton and Obama, has been whether Democratic voters are looking for change.

Winfrey repeatedly proclaimed Obama as the only true agent of change in the contest when she spoke.

"Experience in the hallways of government isn't as important to me as experience on the pathway of life," she said. "What we need is, we need a new way of doing business in Washington, D.C., and in the world."

Clinton's counter-programming included three small events scattered outside Des Moines.