DES MOINES, Iowa – In a rousing speech that most politicians would envy, Oprah Winfrey ignited an overflow crowd by saying her love of country, her alarm over intransigent problems and her hope for national unity has led her to publicly campaign for Democrat Barack Obama.

About 18,500 supporters braved subfreezing weather Saturday to hear the first of four stops in three early voting states that the “First Lady of television” is making on behalf of Obama.

The country is at a critical juncture and needs a leader withconviction and compassion, she said.

“I came out today to tell you why that choice needs to be Barack Obama,” Winfrey said.

Before dozens of TV cameras, and national and international media, the entertainer whose show attracts about 10 million viewers daily said she was nervous about her first campaign tour.

“It feels like I’m out of my pew, out of my terrain,” she said. “But if we continue to do the same thing over and over and over again, I know you get the same results. And it’s time for us all to let America dream again by supporting Barack Obama.”

The crowd was enthusiastic for Obama, but it was ecstatic for Winfrey, who wore a mauve velveteen pantsuit and spoke from a platform backed with a large American flag and a campaign sign that said “Change We Can Believe In.”

Mary Burns, a teacher who traveled 100 miles from Iowa City to attend the rally, said she is still undecided, wavering in support between Obama and Republican Rudy Giuliani. She said that while Winfrey’s endorsement would not affect her vote, it was nevertheless exciting to hear her speak.

Daryl Nelson, a farmer from Menlo, Iowa, said he has long supported Obama. But he had several neighbors calling to beg tickets from him to the rally. “I think they’re taking a more serious look at him today,” Nelson said.

Obama, citing childhood education, health care and the environment problems that he blamed on both Republicans and Democrats, quoted the late Martin Luther King saying there is, “the fierce urgency of now.”

“I need you to stop settling for what the cynics say is possible. I need you to reach for what you know in your heart is possible: a nation healed, a world repaired and an America that believes again,” he said.

As for Winfrey, she said she knew much hype had been attached to her appearance, and she assured the crowd that she knew the difference between endorsing a book and backing a presidential candidate.

She said she knew of Obama’s in the Illinois statehouse, and as a community organizer and civil rights lawyer.

And in a redress to critics, chief among them Hillary Rodham Clinton, who has questioned his experience to be president, Winfrey said, “I challenge you to see through those people who would try to convince you that experience in politics-as-usual is more valuable than wisdom earned from years of serving people outside the walls of Washington, D.C.”

Winfrey said she had voted for as many Republicans as Democrats in her life, so that her maiden stump speech was not a partisan gambit but something, “very, very personal.”

“I am not here to tell you what to think. I am here to ask you to think,” Winfrey said.

When a supporter yelled out that Obama should make Oprah vice president, he laughed. “You realize that would be a demotion, don’t you?” he responded.

Winfrey was also joining Barack and Michelle Obama at stops in Cedar Rapids, Iowa, and Sunday in Columbia, S.C., where the rally was moved to a football stadium to satisfy ticket demand, and at the 13,000-seat Verizon Wireless Arena in Manchester, N.H.