LISA DESJARDINS: Pop culture and politics have a history of crossover, and these intersections take on new significance during an election year when campaigns jockey for endorsements from the entertainment industry's biggest stars, hoping their candidate will latch onto mainstream pop momentum.

LISA DESJARDINS (voice-over): This summer, it was the fluorescent Kamala is brat phenomenon, a meme started by British singer Charli XCX on social media, and then echoed by the Harris campaign and voters across platforms.

100 years earlier, when viral was not a good thing, a different kind of celebrity endorsed the country's 29th president, Warren G. Harding, vaudeville movie star and singer Al Jolson wrote a song especially for him.

Harding, you're the man for us.

MAN: And Mr. Harding, we've selected you.

So it's Harding lead the GOP.

LISA DESJARDINS (voice-over): The idea hit and by 1960 another superstar, Frank Sinatra sang of his High Hopes for a John F. Kennedy presidency.

FRANK SINATRA, American Singer: Everyone is voting for Jack, because he's got what all the rest lack.

LISA DESJARDINS (voice-over): Sinatra's Rat Pack mate Sammy Davis, Jr. threw his weight behind Richard Nixon, and the celebrity role was locked in.

Boxing legend Muhammad Ali endorsed Ronald Reagan, a celebrity actor in his own right.

In 2008 it was Oprah, a beloved multimedia icon making her first political endorsement for Barack Obama.

Eight years later, pop stars Katy Perry and Demi Lovato backed Hillary Clinton.

The 2024 presidential race is no different.

On the last night of the Republican convention in Milwaukee, singer Kid Rock kicked off a big name lineup.

A few minutes later, retired wrestler Hulk Hogan ripped off his shirt to reveal a Trump-Vance logo.

Just a few weeks later, rapper Lil John strode into the Democrats convention in Chicago, one of a bevy of celebrity endorsements.

Actress Eva Longoria and singers Pink and Stevie Wonder lent their voices to the Harris-Walz ticket, the latest in generations of stars giving shine to politicians.

LISA DESJARDINS: But how important are these celebrity endorsements, and do they actually make a difference in changing voters' minds?

Earlier this week, I spoke with Mark Harvey, author of "Celebrity Influence: Politics, Persuasion and Issue Based Advocacy" to help answer that question.

Mark, we're going to talk about whether celebrity, their factor in a race, makes any difference at all.

But first, I want to switch this around.

Why do celebrities do this?

What's in it for them to get involved in politics?

MARK HARVEY, Author, "Celebrity Influence: Politics, Persuasion and Issue Based Advocacy": That's a really complicated question.

First of all, I think you could say that, you know, many of them just have the same impetus as any of us would be.

That is, maybe they feel very strongly about an issue and they just want to share.

Taylor Swift, for example, for a long time wasn't involved in politics, but then she claimed that she got involved because she wanted to be on the right side of history, and so there's that individual aspect.

However, there's always a risk, when they get involved in that, that taking one side or another may also alienate some of their core fan base.

LISA DESJARDINS: We mentioned a major celebrity in the story, Oprah.

Major celebrity, of course, what do we know about exactly how her endorsement mattered for Barack Obama, and is she the rule, or is she the exception?

MARK HARVEY: As far as our what we know she's the exception to the rule.

That's not to say the celebrities don't make a difference at all.

It's difficult to measure.

However, in the specific case, we can demonstrate that Oprah Winfrey probably made the difference in maybe a million votes or so in the Democratic primary, back in 2007 to 2008 that brought Barack Obama to become the candidate.

LISA DESJARDINS: And we know that was a very close primary that Hillary Clinton and Barack Obama went back and forth in the early days.

How did she make a difference?

What is -- what was it that she did that?

Was it just the fact that people admired her?

MARK HARVEY: Yes, I honestly, I think that's a lot to do with it.

She has a certain amount of gravitas.

She already had the so called Oprah effect, where she could endorse a book and suddenly everybody's reading it.

And so, she really had that kind of unique -- that unique credibility at that particular moment.

LISA DESJARDINS: You have said that celebrity endorsements really work when the endorsement kind of matches some aspect of their profile, that there could potentially be a mismatch.

What exactly does that mean?

When does this work?

When does it fail?

MARK HARVEY: Well, let me just give you an example, maybe to put things in context right.

There was a study done a number of years ago with Jessica Simpson.

Well, about Jessica Simpson anyway, and the question is, what is she able to sell, and what the study indicated is that she was quite good at selling perfume, not good at all at selling pocket knives.

And so a lot of this really has to do with image, you know, like what kind of and it's sort of a -- an evasive and elusive thing, but what makes this particular person match up with this product?

That's true for sales of product, but I also would argue it's probably true in terms of selling an issue or selling a candidate.

LISA DESJARDINS: Well, how does that apply to politics?

MARK HARVEY: Here's a good example.

We talked about how Taylor Swift recently endorsed Kamala Harris.

What would happen if she had instead endorsed Donald Trump?

I know that seems a little bit far-fetched, given her predisposition, the way that she stands on certain issues, and we know how she feels about -- LISA DESJARDINS: I'm sure Donald Trump thought about this very question.

MARK HARVEY: Exactly.

And in fact, you know, he created an AI image to try to make it look like, or at least rebroadcast an AI image that suggested that maybe she did support him.

But that's the thing.

All you have to do is watch the Era Tours movie or follow Taylor Swift in the smallest amount, and you realize that she's not a Donald Trump person.

And so there's a little lack there's a break in credibility when somebody does something that just seems against character for that person.

LISA DESJARDINS: Now in your book, you specifically look at issues advocacy, is there a set of issues that really seem to get more traction from a celebrity assist, and some that don't?

MARK HARVEY: Absolutely.

So the ones that are most where celebrities most likely to be persuasive are those ones that are sort of in the Middle that people aren't 100 percent committed to yet.

So maybe certain foreign policy issues, at times, there's a whole host of issues where maybe, if they're not too solid, there's some room.

But if you look at other issues like, say, gun control, the death penalty, abortion, people are really set in on those issues, and Taylor Swift is not going to be able to persuade you that it's time to give up your guns.

LISA DESJARDINS: I know you mentioned a study where people were talked about foreign policy and said, What if a celebrity could influence you on that?

And the answer in that study was yes.

I want to ask you, in terms of the voting groups we're talking about, though, are there different age groups that are more influenced by celebrities, and are these the same groups that vote in large numbers?

MARK HARVEY: Yes.

Well, a lot of research suggests that maybe young people are more influenced by celebrity.

In my research, there was no correlation that it really didn't matter what the age group was.

There wasn't any sort of connection that I could find anyway in terms of age group.

There's some recent research, in fact, some stuff that I've done.

We just did a survey a month ago with 1,000 respondents that suggest that maybe fandom has a lot to do with it.

That is, you have to not only know about the celebrity, but care about the celebrity in order for that celebrity be influential.

LISA DESJARDINS: Mark Harvey, thank you so much.

MARK HARVEY: You are welcome.