Like that other small-town romanticizing heartland-rocker, Bruce Springsteen, John Mellencamp balances pop prowess with bighearted, populist ambitions. Even his grand-old, flag-waving Chevrolet-selling love letter to the working class, “Our Country,” was anchored by a progressive message of tolerance. If that song, with its triumphant accompaniment from country titans Little Big Town, felt like a bid for the radio, there’s nothing remotely commercial about Mellencamp’s latest single, “Jena,” a sorrowful account of Louisiana’s Jena 6 incident sure to divide the pick-up truck set. At this point Mellencamp, who does an 8 p.m. show at the Marcus Amphitheater tonight, seems more interested in ideals than in selling records.