Newt Gingrich’s decision to release his tax returns today indicates he senses that front-runner Mitt Romney is vulnerable as the candidates approach Saturday’s primary in South Carolina. In the first days of the South Carolina campaign, Gingrich challenged Romney’s business record at private equity firm Bain Capital, prompting political analysts to wonder why he would attack a fellow Republican from the Left.

But when Romney gave a faltering answer about his tax returns in a debate Monday night, it bolstered Gingrich’s strategy to pound the front-runner for exploiting the financial system at the expense of middle-class workers.