COLUMBIA, South Carolina (CNN) -- Oprah Winfrey delivered her "favorite" candidate in the presidential race something his campaign hoped for Sunday: the largest crowd yet of any event in the race to '08, according to the Obama campaign.

Oprah campaigned with Obama this weekend in Iowa, New Hampshire and South Carolina.

Although exact figures were not immediately available, campaign officials estimated more than 30,000 people packed into Columbia, South Carolina's Williams-Brice stadium to hear the talk-show queen explain why she believes Obama is the man with the "vision" for America.

"Dr. King dreamed the dream. We get to vote that dream into reality by supporting a man who knows not just who we are but who we can be," she told the crowd. South Carolina is one of the first states in the nation to hold its presidential primary, making it key to the success of any presidential candidate.

Winfrey gave a similar speech Saturday in the first stop of a two-day, three-state tour with her fellow Chicagoan. She discussed on Sunday stepping out of her "comfort zone" by entering the political scene on behalf of a candidate, and praised Obama's "ear for eloquence and tongue for unvarnished truth. We need politicians to tell the truth and be the truth." Watch what issues are important to Iowans »

She also said Obama would bring "a sense of statesmanship" to the White House. Watch Winfrey endorse Obama »

After extensive thank-yous to his wife Michelle and to Winfrey -- and acknowledging that the crowd was largely there to see Winfrey, not him -- Obama launched into his stump speech.

"I am running because of what Dr. King called 'the fierce urgency of now,' " he said.

Covering ground from the Iraq war to the economy to health care, he said, "there is such a thing as being too late -- and that hour is almost upon us."

His campaign worked the crowd, passing out cards to sign up supporters and verifying that those who showed up are registered to vote. They handed out cards asking fans to provide their mailing addresses and phone numbers. The cards also featured an "optional" pledge to vote for Obama on January 26.

In his speech, Obama did not mention his chief rival by name. But he clearly referred to Sen. Hillary Clinton, saying that if he gets the Democratic nomination, his Republican competitor won't be able to say he ever "supported the Iraq war." Watch how Winfrey's campaigning could give Obama a boost »

Clinton was among the Democrats who supported a resolution authorizing force in Iraq in 2002, though in a speech on the Senate floor she said she believed supporting the resolution could ultimately make war itself less likely. Obama was not in the Senate at the time, but gave a speech opposing the possibility of war with Iraq.

Clinton has held a lead over Obama in South Carolina, where the Democratic primary consists largely of African-American voters.

Clinton had a star of her own on the campaign trail for her Sunday in South Carolina -- her husband, former president Bill Clinton, who addressed a church in the morning. Check out some other celebrities who are endorsing candidates »

"She has the best combination of mind and heart and strength of leadership and feeling for ... the problems of ordinary people of anybody I've ever worked with," the former president said Sunday.

Clinton was on the campaign trail with her mother and daughter. E-mail to a friend

CNN's Peter Hamby and Suzanne Malveaux contributed to this report.

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