Listen close after a Mitt Romney rally, and you’ll hear a soundtrack with a very un-Romney-like tone.

Romney’s main theme song for this campaign is a bland, upbeat anthem: “Born Free,” by Kid Rock.

But, after his speeches, Romney plays music with an angrier edge.

“Winter gettin’ colder. Summer gettin’ warmer. Tidal wave comin’ ’cross the Mexican border,” country artist Toby Keith now sings over Romney’s loudspeakers at some rallies.

The campaign has started using the song “American Ride,” which channels blue-collar unhappiness about immigration, gas prices and political correctness. (“Don’t get busted singin’ Christmas carols.”) It has a sardonic tone that seems out of sync with the genial, G-rated Romney himself.

As does the cursing.

“Daddy works his ass off, paying for the good life,” the speakers boom. Then: “Hot dog! Hot damn!”

None of this year’s GOP candidates really seem to have mastered the art of the campaign song. Former House speaker Newt Gingrich (Ga.) blares “Independence Day,” a tune about domestic violence and death-by-arson. Texas Rep. Ron Paul once took the stage to the menacing dum-dum-dum-dum-da-dum of Darth Vader’s theme song. And former senator Rick Santorum (Pa.) sometimes uses no music at all: When his rallies end, an awkward silence descends.

In Romney’s case, his campaign playlist has already traced a shift in his political persona — from 2008’s Boston businessman (playing the Fenway Park standard “Sweet Caroline”) to 2012’s untethered American conservative (“like an untamed stallion,” Kid Rock sings).

Now, the music seems to show Romney reaching out to the right-wing voters who have spurned him so far.

“There is no chance that these songs are chosen at random,” said Benjamin S. Schoening, a professor at the University of Wisconsin at Barron County who co-wrote a book on presidential campaign songs. “If you’re playing songs that have these very conservative messages within them, you can set a mood for the audience that you are actually a conservative candidate — without actually having to say it in your stump speech.”

The campaign song is almost as old as the presidential campaign itself. Supporters of John Adams sang “Adams and Liberty,” set to the tune that would later become “The Star-Spangled Banner.”

In modern times, however, candidates have generally turned away from custom-written tunes. (1964’s “Hello, Lyndon,” set to the tune of “Hello, Dolly,” may have been the beginning of the end. Or maybe it was 1972’s oddly hippie-ish “Nixon Now.”) They repurposed old pop songs instead.

In the best case — as in Bill Clinton’s “Don’t Stop” — these familiar hits can fire up a crowd and send supporters home with the candidate’s tune on mental repeat. In the worst case . . . well, the worst case is probably Ross Perot. He chose Patsy Cline’s “Crazy” in 1992.

“The best ones, really, capture some message about the candidate — that really distills [the candidate] to an essence — and then gets people excited about it,” said Mark Clague, a professor of musicology at the University of Michigan. The music also allows a candidate to send a message to voters without actually saying the words.

“They can always say somebody else chose it,” Clague said.

When Romney first ran for president, in 2008, he used Boston songs such as “Sweet Caroline,” and “Dirty Water,” by the Standells. To highlight his promise to break Washington’s gridlock, Romney played Elvis Presley’s “A Little Less Conversation.”

This time around, Romney has a new songbook. His campaign did not respond on the record to a request for comment this week, asking about the reasoning behind Romney’s current campaign songs.

But a few things are obvious. One is that there is no trace of Boston left.

Instead, “Born Free” sounds vaguely western, talking about mountains and canyons and rivers raging. Its lyrics mention a dominant theme of the tea party movement — personal freedom — without getting into specifics.

Romney also recites another song in his stump speech: “America the Beautiful.” He usually repeats parts of three stanzas, using each to make a political point. The part about “O beautiful, for heroes prov’d, in liberating strife” takes him to, “Do we have any veterans in the room?”

“O beautiful, for patriot dream, that sees beyond the years.” That leads to a practiced bit about the Founding Fathers and how President Obama doesn’t understand them.

And the first stanza, with its description of “purple mountain majesties,” becomes a chance for Romney to repeat blanket declarations of love for America.

“This is a wonderful place,” Romney told a crowd in Sparks, Nev., a few days ago. “I love America.”

After Romney finished that speech about America’s virtues, the loudspeakers began blaring a fiddle riff.

And then Toby Keith came on to talk about the country’s flaws.

“Plasma gettin’ bigger, Jesus gettin’ smaller. Spill a cup of coffee, make a million dollars,” Keith sang in “American Ride.” At other events, Romney has also played a Keith song called “Made in America,” which describes an older man unhappy with the ways that America has changed: “It breaks his heart, seein’ foreign cars filled with fuel that isn’t ours, and wearin’ cotton we didn’t grow.”

Keith’s words and rowdy tone may have resonated with some conservative voters. But it didn’t sit well with Mary Kronvall of Grand Junction, Colo. After she heard “American Ride” played at a Romney rally in her home town this week, she approached a group of reporters to complain.

“I’m a born-again Christian, and I don’t believe in swearing,” Kronvall said in a telephone interview later. In particular, she said, it was the words “Hot damn” that bothered her.

“And it was to rhyme with American,” she said.