After the auditorium fills to capacity, hundreds push against the registration tables outside, waving their arms for pins and yard signs to be tossed their way. “We’re not gonna leave!” shouts student Chris Larez, demanding a glimpse of the candidate. The overflow crowd chants: “Beto! Beto! Beto!”

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“If you continue to charge the table, we will stop giving out buttons,” a nervous volunteer says over the loudspeaker.

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Such a frenzy, though common for O’Rourke (upwards of 50,000 came to his event with Willie Nelson), may have no precedent in a Senate race. But in a sense, Betomania is a cover of a much older tune: Robert F. Kennedy’s campaign, preceded by his poverty tour through Appalachia. “Someone gave me a book on his ’68 campaign,” O’Rourke tells me, and what he read about inspired him, especially Kennedy’s time with those “who were counted out or forgotten.”

That inspiration is a template for O’Rourke’s mission to prove that the supposed trade-off facing Democrats — fire up the partisan base or appeal to independents and Republicans alarmed by President Trump’s excesses — is a false choice. His shoe-leather campaign through the 254 Texas counties aims simultaneously to boost turnout among the young and minorities, while also to compete in Republican strongholds full of white Trump voters worried about their way of life. “People look at us like maybe we read the map wrong,” he quips.

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O’Rourke faces long odds. If somehow he prevails, he would vault to the front of the pack of Democratic presidential hopefuls in 2020. But, win or lose, he seems to have a formula for exciting partisan Democrats without spooking everybody else.

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Democrats in competitive House and Senate races across the country, trying to appeal to voters fed up with hyperpartisanship, are trying some version of this, emphasizing their desire to “work together.” But none has found a special sauce anywhere near as popular as the one flavoring Betomania.

His speeches have little red meat and few mentions of Cruz or Trump. And though he doesn’t hide his progressive positions — for single-payer health care and legal marijuana, against a border wall — he avoids purity tests, such as a reflexive demand for Trump’s impeachment.

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His partisan jabs are delicate. Speaking in San Antonio about the border wall, Trump’s family separation policy, his attacks on the media and his travel ban on several Muslim countries, O’Rourke said he doesn’t want future generations “to look back and say, ‘Who were those pendejos?’ ” The audience roared at his use of a Spanish profanity to declare the president stupid.

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Harder for other Democrats to reproduce is O’Rourke’s cool factor: skateboarding at Whataburger, playing the air drums, doing his laundry on Facebook Live, and scoring appearances with Ellen DeGeneres and Stephen Colbert after saying there’s “nothing more American” than the right of National Football League players to protest during the national anthem.

And, certainly, a lot of Betomania has nothing to do with O’Rourke: He’s the vessel for people appalled by Trump and by Cruz, one of the nastiest in the business. O’Rourke argues, correctly, that people have already “formed an opinion” about those men and that there’s not “much I can add to that.” This allows him to rise above Cruz’s endless falsehoods and venom.

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His stump speech is a bit sleepy (college affordability, his refusal to take PAC money, the threat to farmers from climate change), and he labors to play down his affiliation. “I could care less, to be honest with you, about Democrats, or about the Democratic Party,” he says in a video interview on screen before he speaks.

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Most evidence suggests the grip of partisanship has never been stronger. O’Rourke believes the opposite, that the trauma of the moment “is allowing people to transcend the conventional definitions by party.”

At least it’s allowing him to, judging by the thousands who routinely show up to see him, and fundraising that dwarfs Cruz’s.

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When O’Rourke finally finishes his speech in San Antonio, to earsplitting cheers, he emerges to see hundreds waiting for pictures with him. Police form a ring around him to push back the surging crowd, to no avail.

Had the reaction been like this two decades ago when O’Rourke toured the country as a little-known punk rocker, he never would have gone into politics. “It was nothing like this,” he says, waving to the throng. With a kamikaze yell — “heyyyyyyyyyy!” — the candidate plunges into the crowd, and a roar goes up.

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Twitter: @Milbank