Bill Clinton's 1992 presidential campaign had a winning musical message. When he chose Fleetwood Mac's "Don't Stop" as his unofficial theme song, it became the sound track for a political moment. It captured a certain optimism, which in turn helped capture high office for the man smart enough to approve this message.

Two and a half decades later, the music is all about yesterday.

The current pack of presidential hopefuls is using a range of strategic sounds to reinforce messages, but the odd-couple "outsider" campaigns - those of Bernie Sanders and Donald Trump - are strumming heartstrings that yearn for the better America they say used to be.

What exactly was better, and for whom, depends on what you hear in these sounds.

For Trump, it's 1942 again - or even 1917. At a rally last month in Pensacola, Fla., three members of the band USA Freedom Kids, young girls in tops of stars and skirts of stripes, sang and danced their way through a tune recalling "Over There," George M. Cohan's call to action used in both World Wars.

"Enemies of freedom, face the music. C'mon boys, take 'em down!" sing Trump's trippy trio.

Youth belting out a bellicose message - in videos widely circulated on social media - reminds some of the kind of jingoism more often emanating from North Korea.

The performance is memorable for its "overt bizarreness," says Wharton School marketing professor Americus Reed. "Here, Trump is just doing what he does really well. He's trying to do something so odd, it stands out and allows him to once again hijack airtime," says Reed, a brand-identity theorist (and amateur drummer). "In the end, it creates the ability to be perceived as out front and always on people's mind - even if for the wrong reason."

Sanders has a sweeter view of America. His "America" spot, using the Simon and Garfunkel song from 1968, carries a tagline - "A Future to Believe In" - that speaks of what's to come. But the acoustic guitar and accompanying shots are unmistakably of an America remembered.

The popularity of the song holds nostalgic appeal and relevance for a broad audience, says Amy E. Jasperson, associate professor and chair of the political science department at Rhodes College in Memphis. "The song starts out slowly while the ad shows individual images of small towns, urban landscapes, ordinary people, farmers, and families," she says. "As the song builds, the people are brought together. By the end of the ad and the song, the viewer hears and sees the crescendo of huge, cheering, unified crowds."

That sense of unification is intentional, Jasperson says, and it has proved popular, getting the spot nearly three million YouTube views in a little more than two weeks.

"Not only does this song hold nostalgic meaning for those who first heard the song in their youth; the aspirational lyrics and tone also reference a common search for where people feel we should be going as a country," she says. "This ad suggests that Sanders can lead people to that answer."

Campaigns sometimes rely heavily on music's emotional impact when trying to appeal to voters who are dissatisfied, says Eric T. Kasper, assistant political science professor at the University of Wisconsin-Eau Claire.

"There are multiple candidates this year who are running on a message that the 'system is broken' or 'Washington isn't working,' so it makes sense that music is more of a focus," he says. "We've seen emotional appeals like this during elections at times when trust in government is low, like it is now."

He points to 1972, during the Vietnam War, when the George McGovern campaign used Simon and Garfunkel's "Bridge Over Troubled Water," which he says was one of the first times a presidential campaign used a pop song.

All of the hopefuls are using music to varying degrees, but Hillary Clinton, Sanders, and Trump are taking a more personalized approach. Clinton asked supporters in 2008 to participate in an online poll to help her choose her official campaign song, and this time around, she is aiming younger with help from Demi Lovato and Katy Perry.

In 2012, Barack Obama and Mitt Romney released their Spotify playlists, notes Kasper.

"Trump, being the showman he is, has played multiple songs at rallies over the past few months to help reinforce his different messages," says Kasper. "I think you can see this in messages that range from promoting freedom/patriotism with Neil Young's 'Rockin' in the Free World' - which is actually a song that discusses what Young saw as problems in the country - to portraying himself as a visionary with Aerosmith's 'Dream On,' to questioning whether Ted Cruz qualifies as a natural-born citizen with Bruce Springsteen's 'Born in the U.S.A.' This isn't to say that other candidates aren't trying to do similar things, but these three campaigns seem to be doing a better job of paying attention to music."

Trump has made liberal use of the intellectual property of others without permission. When his campaign played Adele's music at rallies, the singer released a statement saying it had been without her consent. Aerosmith front man Steven Tyler had his attorney send the Trump campaign a cease-and-desist letter.

Although viewers of political ads and rallies may not always be fully conscious of the music, it is an incredibly efficient way of communicating the message.

"Music has a way of just eliciting an immediate response," says John Baker, sound designer with Philadelphia Post, a Center City video and audio house that works on political ads. "It's the quickest way to get somebody to feel afraid, to make them feel good. There is even a category within stock music called dark to light - it starts out with, 'this is the bad person,' and then goes to 'this is the good person.' "

Marco Rubio, Jeb Bush, and Chris Christie tend to use generic music, often electronic sounds simply conveying momentum, importance, strength, or energy. Fearmongering music has a star role in several current ads.

One spot, called "Rebuild Our Military, Kill the Terrorists," stands apart. It was highly rated among Republican viewers surveyed by Ace Metrix, an analytics company used by several of the campaigns.

In it, Cruz looks into the camera and tells viewers that the world is getting more dangerous and that "under no circumstances will I ever apologize for America."

The sound track Cruz chooses as emotional reinforcement behind his own voice? Dead silence.