Democratic presidential candidate Sen. Barack Obama, D-Ill., speaks as his wife, Michelle, left, and Oprah Winfrey, center, listen during a rally Saturday in Des Moines, Iowa.

Des Moines, Iowa ? Talk-show diva Oprah Winfrey said worry about the direction of her country and a personal belief in Barack Obama pushed her to make her first endorsement in a presidential campaign, invaluable support in a tight race for the Democratic nomination.

The weekend “Oprahpalooza” lends A-list star power to Obama’s campaign, drawing huge crowds that Obama hopes will translate into votes. Tens of thousands were expected to turn out for Winfrey’s Iowa stops and her visits today to South Carolina and New Hampshire with Obama and his wife. In South Carolina, the campaign ran out of the 18,000 tickets originally available for the biggest event and moved it to the 80,000-seat University of South Carolina football stadium.

In Iowa, spectators lined up hours early. Cameras flashed in the capacity crowd during Winfrey’s speech, which opened and closed to loud applause and was frequently interrupted by cries of “We love Oprah.”

Winfrey said she felt nervous and “out of my pew” as she addressed a gathering hall packed shoulder-to-shoulder in the largest gathering of Iowans in the campaign this year. But she did not hide her political convictions, making an argument for change from the Bush administration other than another Clinton in the White House.

Winfrey did not mention the current president or Obama rival Hillary Clinton by name, but was not subtle about her feelings for Clinton’s argument that Obama doesn’t have the experience to be president when she voted to authorize the war in Iraq.

“The amount of time you spend in Washington means nothing unless you are accountable for the judgment you made,” Winfrey said. She said from the beginning Obama “stood with clarity and conviction against this war in Iraq.”

“There are times that I even worry about what happens to our country,” Winfrey said, standing on a small stage before a sea of people in the 100,000-square-foot hall. “That is why 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.”

The campaign distributed 23,000 tickets for the Des Moines event and more than 10,000 for another later in Cedar Rapids. Thousands of people, many who don’t normally participate in politics, came into his offices, volunteered and attended caucus trainings to score tickets.

The campaign said 18,500 people showed up in Des Moines. At least one person near the stage passed out, and paramedics came in to help.

The Democratic race in Iowa is tight, with Obama, Clinton and 2004 vice presidential nominee John Edwards in a dead heat.