Oprah Winfrey electrified an Iowa audience Saturday as she hit the stump for Barack Obama, telling the crowd she decided to make her first endorsement in a presidential campaign out of a belief that Obama has a "new vision" for the country.

The billionaire talk show host, whose emphatic address was frequently interrupted by applause and cheers from likely fans, came to the Des Moines rally as part of a three-state tour with the Illinois senator. As she, Obama and Obama's wife Michelle kicked off the events, Obama's top rival Hillary Clinton also swept through Iowa, at stops with her daughter, Chelsea, and 88-year-old mother, Dorothy Rodham.

Clinton often campaigns on her experience in Washington, D.C., but Winfrey was not afraid to take a subtle swipe at that argument Saturday. She told the Des Moines audience that "the amount of time you spent in Washington means nothing unless you're accountable" for your decisions.

"For the first time in my life, I feel compelled to stand up and speak for the man that I believe has a new vision for America," Winfrey said. "I am not here to tell you what to think. I am here to ask you to think, seriously."

Winfrey said she was nervous and felt out "of my terrain," but that she didn't make any predictions for the effect her endorsement would have on the race.

"So much has been said about what my jumping in to this arena does or does not bring to the table of politics. I really don't know," she said. "Despite all of the talk, the speculation and the hype, I understand the difference between a book club ... and this critical moment in our nation's history."

Obama hopes the talk show host's star power will lend him access to a broader cross-section of voters. 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.

Winfrey and the Obamas flew through snow to the Cedar Rapids event, though weather grounded Obama rival Joe Biden's campaign plane traveling the opposite route about the same time. Temperatures were in the low teens, and the Obama event was not all the way full.

Winfrey delivered much the same speech that she did in Des Moines, again drawing cheers as she read prepared remarks. Winfrey drew laughter as she joked that she wasn't there to give out free cars and refrigerators as she has on her daytime talk show.

"Over the years, I have voted for as many Republicans as I have Democrats," Winfrey said — one line that didn't draw applause in the partisan crowd. "This isn't about partisanship for me. This is very, very personal. I'm here because of my personal conviction about Barack Obama and what I know he can do for America."

She said she is "tired of politics as usual," which is why she seldom invites politicians on her show to spread their rhetoric. Obama, she said, has an "ear for eloquence and a tongue dipped in the unvarnished truth."

The campaign said 18,500 people showed up in Des Moines.

From Iowa they head to New Hampshire and South Carolina.

The campaign just announced that Arrested Development will headline the pre-program show in Columbia, S.C., on Sunday. The event had to be moved earlier in the week from an 18,000-seat basketball arena to a football stadium that holds 80,000, just to accommodate the crowd. The campaign doesn’t expect to fill Williams-Brice Stadium, but officials are anticipating big numbers. The campaign ran out of the free tickets for the old venue just two days after it began distributing them — tickets will no longer be needed.

The Obama campaign has said it’s filled its Sunday event with Winfrey at the Verizon Wireless Arena in Manchester, N.H., which holds close to 12,000 people.

But Clinton has been heading off the much-anticipated Oprah tour at every turn. Her mother-daughter team accompanied her Saturday, and Bill Clinton campaigned for his wife in South Carolina as well, promoting her health care plan, one day before Winfrey arrives with Obama.

"We're getting close to the caucuses," Hillary Clinton said. "I always think it's better to go to the caucuses with a buddy. Today, I've got some buddies with me."

As for Obama's buddies, the question is whether Winfrey's immense popularity will translate into votes. The media mogul and talk-show host, who can single-handedly anoint a writer into a best-selling author, has never endorsed a presidential candidate, but she's gone headlong into this endeavor. A glitzy fundraiser held at her California estate in September raked in $3 million for the candidate.

Asked about the Oprah factor at a deli in Des Moines, Clinton gave a non-answer.

"I'm having fun. I'm not going to eat this whole thing with all of you watching, though," she said while eating breakfast with her mother and daughter.

Clinton, who has been endorsed by Barbra Streisand, later said celebrity endorsements are just one factor voters consider.

"At the end of the day it’s a choice among those of us who are running," she said. "And I think most voters understand that, and they sort it all out. They make up their decisions."

Clinton said her daughter's appearance on the campaign trail was already planned far in advance of this weekend, and defended her record in office in response to Winfrey's experience comments.

Obama said Saturday he was grateful to have Winfrey in his corner.

"You want Oprah as vice president?" he asked the crowd. "That would be a demotion, you understand that?"

A Pew Research Center survey conducted in September showed that 60 percent of those polled thought she would help Obama's candidacy — but only 15 percent said they were more likely to support Obama because of her. Sixty-nine percent said Winfrey would have no effect on their vote.

Former Iowa Gov. Tom Vilsack, a national co-chairman for Hillary Clinton, has brushed off the Winfrey appearances.

"I'm not sure who watches her," he told The Washington Times in late November. "Maybe young moms, maybe people who are retired. But we have the support of most retired Democrats."

But Obama is creeping up on Clinton.

A recent ABC/Washington Post poll of 592 likely voters conducted from Nov. 29 to Dec. 3 showed Clinton's lead in New Hampshire diminishing. The poll showed Clinton with 35 percent and Obama closing in with 29 percent.

Former North Carolina Sen. John Edwards pulled 17 percent in the poll. If she can keep her lead, the Jan. 8 Granite State primary would be a potential fallback for Clinton should she fair poorly in Iowa on Jan. 3.

Recent Iowa polls show Clinton, Edwards and Obama close together at the top, with Obama leading in several.

A new AP-Ipsos poll, however, showed virtually no change nationally in the Democratic race from last month. In the survey, Clinton kept about a 2-to-1 lead over Obama, 45 percent to 23 percent, with John Edwards at 12 percent. The poll interviewed 1,009 adults nationally and was conducted from Dec. 3 to Dec. 5.

FOX News' Molly Henneberg, Aaron Bruns and Bonney Kapp and The Associated Press contributed to this report.