Sergio Massa marches down a corridor, casting aside his suit jacket and rolling up his shirtsleeves – as if preparing for a schoolyard tussle – before facing the camera: “If they want to fight, we’re going to fight,” he says. That’s the controversial TV spot Mr. Massa, who is running for a congressional seat in Argentina’s upcoming midterm elections, chose for his campaign. But he is not the only politician to adopt an aggressive tone against the Front for Victory, President Cristina Fernández de Kirchner’s ruling alliance.