This March, Billy Bragg took the stage at a festival in Florida, alone with his electric guitar. He tore into “To Have and To Have Not,” an explosive song from his first album about a working-class British kid trying to find a job in a recession, fighting the sense he’s one of capitalism’s castoffs. Months earlier, a quieter Bragg stood in a Spanish radio studio and performed “She’s Got A New Spell,” a clever play on the idea of love as magic: his new girlfriend’s a witch, mumbling spells in Latin, bending the law of gravity, setting his room a-twirl.

Two very different, original personas co-existed on Bragg’s classic early albums. The protesting socialist, a “one-man Clash” shouting down the hypocrisies and arrogance of the powerful, duetted with a romantic charmer armed with wit and wordplay. Those two sides of Bragg’s talent also shone through on his Mermaid Avenue collaborations with Wilco, which interpreted once-lost Woody Guthrie songs. Bragg wasn’t just a natural inheritor of Guthrie’s folk protest tradition; his shambling renditions of sly lyrics such as “Walt Whitman’s Niece” revived an earthier side of Guthrie: the winking cad.

The last ten years have challenged Bragg as a songwriter. He faces the mature rock troubadour’s eternal dilemma: it’s much easier to write a great breakup song or ode to new love than a compelling song about a decades-long relationship. It’s also harder to rouse an audience with fiery protest songs now that Bragg doesn’t have Margaret Thatcher to kick around anymore and Britain’s politics have become splintered and centrist.

But there are plenty of reasons why Bragg’s September 11 visit to the Ark has sold out. Though his last two albums have been uneven, a Bragg gig has always been more than a concert: it’s part rally, part benefit, part forum. That’s even truer now that Bragg has evolved into a public intellectual and author of some stature in Britain. Seeing him today is a bit like going to a talk by Gloria Steinem or Cornel West. He’s likely to chat between songs about this year’s British election results and about Jail Guitar Doors, a charity he founded that donates guitars to prison inmates for rehabilitation and music therapy.

He’s also likely to perform “I Keep Faith,” the best song from his 2008 album, Mr. Love and Justice, in which he confronts the challenges of romantic and political commitment. He could be singing to his partner or to anyone who shares his belief in social change: “If your dreams should come to nothing/Washed out in the rain/Let me rekindle all your hopes/And help you start again.” Bragg, twenty-seven years into his career, has kept faith, thinking harder than anyone else about the limits and possibilities of mixing pop and politics. He is the purest musical conscience of his generation, and by conscience I mean the song in your head that reminds you what you believe in.