When it comes to purses, Japanese coffee and cologne, getting an endorsement from the likes of Kate Moss, Brad Pitt or Beyoncé can make all the difference in the world. But are Iowa caucus-goers really more likely to jump onboard with Barack Obama or Hillary Clinton on January 3 because Oprah Winfrey or Katharine McPhee have given them their respective thumbs up?

Whether star-power endorsements make any difference or not, when talk-show mogul Winfrey took to the stage to stump for Obama recently in Iowa and South Carolina, she drew massive media attention and thousands of supporters, adding to the Illinois senator's pool of marquee names, which already included Will Smith and rapper Master P. Smith has said of Obama, "I think he's what the future of America is going to be," while fellow Hollywood star Halle Berry has been photographed wearing a T-shirt stating "Obama for Change." And recent concerts in support of Obama have included sets from John Legend, Ne-Yo, Wilco's Jeff Tweedy and the Goo Goo Dolls.

"For general elections, [celebrity endorsements] tend not to matter, because there are only two serious candidates, so an endorsement at that final stage won't make much of a difference," said David King, a lecturer on public policy at Harvard University's John F. Kennedy School of Government. "But in a crowded primary field, endorsements matter the most, and both the Democratic and Republican fields are very crowded [with candidates] now. If [an endorsement] could make any difference, it would make one now."

King said there hasn't been a concrete political-science study on the power of celebrity endorsements in the race for president, and because Winfrey — whose support has turned authors and products into best-sellers in the past — has never supported a candidate before, it's unknown what her imprimatur really means.

The closest anyone has come to documenting the importance of the star surge was in 1968, when Richard Nixon's candidacy got apparent help from the support of movie tough guy John Wayne. "The short answer," King argued, "is that it depends on the state and constituency."

One professor who has studied this phenomenon, Natalie Wood — a marketing expert at Saint Joseph's University in Philadelphia — said the audience most often targeted by these vote-chasing stars is young, first-time voters. And they're exactly the ones who might be least willing to listen.

"This is a generation that has grown up in front of the computer and TV screen, and they are very skeptical and critical of any advertising message," said Wood, co-author of a recent report that looked at the 2004 presidential election. "Political Star Power and Political Parties: Does Celebrity Endorsement Win First-Time Votes?" came out this year in the Journal of Political Marketing. "[Young voters] can see through what celebrities are attempting to do and recognize that they're often doing it to promote and market themselves," Wood said.

According to Wood, the thing celebrities like Winfrey are good at is drawing attention to candidates — but when push comes to shove, it's friends, family and peers who have the most influence in swaying votes. "If I happen to like a particular candidate and a celebrity is endorsing them, it only reaffirms my feeling that I'm right," Wood said. "But if a celebrity says something bad about a candidate, what do I care? It only reinforces my existing beliefs, and it won't change my mind."

Over the past few weeks, the endorsements have been coming fast and furious. Senator Clinton's list includes Barbra Streisand, poet Maya Angelou and Steven Spielberg, as well as a younger generation of entertainers, among them McPhee. While performing at a Clinton-sponsored event in the spring, the "American Idol" alum gave props to the New York senator, saying, according to a press release on the candidate's Web site, "It's important for all of us to get involved in the political process, and I'm happy to do my part by being part of this great event and performing for our next president, Hillary Clinton." Legendary names such as director Rob Reiner and record producer Quincy Jones have also starred in ads posted on Clinton's campaign Web site, and on Tuesday, former NBA great Magic Johnson showed up at a Hy-Vee grocery store to put in some face time with Clinton.

And while he didn't officially endorse the senator, 50 Cent expressed his appeal for Clinton in a Time magazine article in September, stating, "I like Hillary. I think she was already our president once."

Democratic candidate John Edwards has also gotten some help in Iowa and New Hampshire recently, from veteran singers Bonnie Raitt and Jackson Browne, as well as actors Kevin Bacon and Tim Robbins. Republican Senator John McCain has launched an ad campaign featuring Boston Red Sox pitcher Curt Schilling, and in perhaps one of the strangest bits of synergy between Hollywood and Capitol Hill, suddenly red-hot Republican candidate Mike Huckabee has been appearing alongside action hero Chuck Norris in a series of TV ads.

Wood said the tide of celebrity endorsements just appears to be getting bigger and bigger with each presidential election, but even some of the stars doubt how much impact they are really having.

"I read where Kevin Bacon said he wasn't sure if he will make a difference, but he wanted to do something," she said.