MICHEL MARTIN, HOST:
Now, big rallies like this are certainly exciting. But what might also help these candidates get through the primaries are endorsements from celebrities, like Katy Perry, and from other big-name politicians and leaders. But when it comes to endorsements, which ones really matter? NPR's Sam Sanders went looking for answers.
SAM SANDERS, BYLINE: There was super-producer Pharrell Williams on "The Ellen Show" recently.
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PHARRELL WILLIAMS: I just feel like it's Hillary time.
ELLEN DEGENERES: It's Hillary time?
WILLIAMS: Yeah.
DEGENERES: Wow. That's great.
SANDERS: And there was the "Duck Dynasty" guy, Willie Robertson, endorsing Donald Trump.
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UNIDENTIFIED MAN: Willie, do you love Trump?
WILLIE ROBERTSON: I do like me some Trump, I've got to admit.
SANDERS: Stars endorsing presidential candidates - this is a big deal, right? Well, actually, no.
LYNN VAVRECK: It's not celebrity endorsements that predicts who the eventual nominee becomes.
SANDERS: That's Lynn Vavreck, a political scientist at UCLA. She's found that celebrity endorsements are really just meant to get people talking.
VAVRECK: What's highly correlated with who becomes the nominee is the number of party elite endorsements a candidate has in the year before the election.
SANDERS: Party elites - politicians, governors and senators and party chairs. Here's why.
VAVRECK: The idea is not that anybody hears that someone has endorsed you and then that sways their vote. The idea is that party elites have a sense of who is viable and electable as a candidate. They have unique insider knowledge about this.
SANDERS: More knowledge than a celebrity. So this kind of endorsement for Clinton from Housing and Urban Development Secretary Julian Castro, it might be worth more than Katy Perry's. Here's Castro's endorsement from MSNBC.
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JULIAN CASTRO: I believe that she has a very strong vision for the future of the country.
SANDERS: And Stuart Stevens says more of those endorsements will come further down the road.
STUART STEVENS: It's better to get an endorsement when people are paying attention. It's like a Christmas sale - having a Christmas sale in August isn't as good as having one two weeks before Christmas.
SANDERS: Stevens was a top strategist for Mitt Romney's 2012 run. He says as candidates continue to rack up endorsements, there's one thing you really want.
STEVENS: Does it seem surprising? You know, if your mom endorses you, it's probably not a surprise. If your mom endorsed your opponent - that would be a surprise.
SANDERS: Maybe Bernie Sanders should try to get an endorsement from, say, Julian Castro's brother, Joaquin, or Jeb Bush should trot out Joe Biden. That would be pretty exciting. Sam Sanders, NPR News. Transcript provided by NPR, Copyright NPR.
