Melanie White wasn't paying much attention to the presidential campaign. But when she heard Oprah Winfrey was coming to Des Moines to campaign for Barack Obama, politics suddenly mattered. She wanted to see Oprah.

Her friend Kim Smith, a committed Obama supporter, told her she could get tickets, but there was a price. "She has to sign her life away to volunteer and caucus for Barack," said Smith.

White readily agreed. And so the two 30-something friends sat near the front of a line of more than 18,000 waiting to get into the Hy-Vee Hall in downtown Des Moines, a copy of "O's Guide to Life" and an "Obama '08" bumper sticker between them.

Call it the "Oprah effect," a phenomenon the political world is watching warily. Not because celebrity endorsements are new but because Winfrey is more than a celebrity: She's a social icon, an earth mother, a television priestess of sorts whose predominantly female flock takes her words to heart.

"The problem with most celebrity endorsements is that there's no transferability between their talent and real credibility," says Howard Davidowitz, chairman of Davidowitz & Associates, a retail investment banking firm. "Oprah is different. Oprah has an army out there that really listens. She's one of the great marketing machines in history."

But politics isn't soap powder. And as Winfrey rose to the podium in the packed convention hall to stump for a presidential candidate for the first time in her life, the first lady of television made it clear Saturday that she knows the difference.

"Despite all of the talk, the speculation, and the hype, I understand the difference between a book club and free refrigerators ... and this critical moment in our nation's history," she says. "I came out here for, I suspect, the same reason you did: Because I care about this country."

Celebrity endorsements have been a popular political tool for nearly a century. The government, for example, hired Charlie Chaplin to help sell war bonds in 1918, notes Steven Ross, a history professor at the University of Southern California in Los Angeles.

"Historically, the real power of celebrities in politics has been getting people to show up for events," he says. "Once they're there, they listen to what the candidate has to say."

Some are getting earnestly involved in the political process because of Winfrey's call to action. Jacqueline Pope and her sister-in-law Sandra Pope drove 90 miles from Ottumwa, Iowa, to be at the Des Moines rally. To them, it was a "package deal."

They've supported Obama for some time, but now with Winfrey's endorsement, they're determined to go to the caucus on Jan. 3. It's only the second time in the 18 years Jacqueline has lived in Iowa that she will have gone to a caucus. Sandra, who's lived here just as long, will be going to her first caucus.

"He has a vision, and it's about hope for our country that right now is in very serious trouble," says Jacqueline.

The Popes are two of Winfrey's estimated 8.6 million viewers, and they represent a crucial demographic, says Ross. Two-thirds of them are women, and nearly half make less than $40,000 a year, according to Nielsen Media Research.

Many of them are probably not registered to vote, Ross says, and Winfrey could very well get them to the polls. "She could tap in to the 50 percent of the population that doesn't vote," he says. "When Oprah says, 'This is somebody I really support,' she has the potential to reach out to voters who never vote."

But not everyone is convinced that the "Oprah effect" will draw in a significant number of new voters. Dennis Goldford is a professor of politics at Drake University in Des Moines.

"For a good 40 years now, campaigns have tried to market candidates as if they were soap powder or breakfast cereal," he says. "But I don't think people yet blur the line between citizen and consumer."

Margaret Blair is one such rallygoer. "I like Obama, too, but I'm especially here because Oprah came with him," she says. "I haven't really decided yet whom to support."