Barack Obama hopes to light up the campaign trail this weekend with four high-profile stops across three early voting states featuring the envy of endorsement seekers.

Oprah Winfrey will lend her star power to the Obama campaign once again in a tour that is already selling out. The stops come as Obama begins to close in on frontrunner Hillary Clinton in the states the duo visits.

The rallies kick off Saturday in Iowa, with events in Des Moines and Cedar Rapids.

And a planned rally Sunday in South Carolina had to be moved earlier this 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.

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 before — though she has interviewed them on her show in a less partisan setting.

"I think the real significance of Oprah is that her appeal crosses racial lines, just as Barack Obama's appeal — or he hopes his appeal — crosses racial lines as well," said political analyst Michael Barone.

Winfrey's gone headlong into this endeavor. A glitzy fundraiser held at her California estate in September raked in $3 million for the candidate.

"Oprah is a great friend," Obama said at a stop in New Hampshire last month. "Obviously she is beloved across the country. Ultimately people are going to make a decision ... based not on who's endorsing you, but people will certainly come to an event to see Oprah, and that means that I've got access to more people and hopefully can tell them the kind of change that I want to bring about."

Pamela Gentry, BET News Washington bureau chief, said the Oprah factor gives Obama publicity, but not necessarily votes.

"I don't know that that can translate into a voting booth," she said. "Again, having a best-selling book or having a robe that sells out at a department store because she calls it her favorite is a little bit different, I think, than picking a presidential candidate."

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.

The Oprah tour, though, only escalates the battle of surrogates that has waged in the run-up to the early primaries.

Around the time this weekend’s tour was announced, Clinton rolled out the endorsement of Barbra Streisand. And she’s already had a big helping hand from a political rock star: her husband.

Trying to counter the Oprah effect, Bill Clinton plans to campaign for his wife in South Carolina Saturday, one day before Winfrey arrives. Obama will also be appearing with his wife Michelle this weekend.

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."

Oprah or no Oprah, Obama's 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, as Obama takes the lead in some polls in Iowa.

The New Hampshire 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 Jan. 3.

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. 3-5.

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