In state after state, exit polling has shown a clear pattern in the Republican presidential primary: Mitt Romney performs worst among low-income whites and those without college degrees, groups who form a core part of the Republican base. Super Tuesday’s biggest prize, Ohio, may prove his toughest test yet, as Rick Santorum leans heavily on his background as a “kid from a steel town’’ in an all-out bid for the support of working-class voters in that Rust Belt state. White working-class voters - those without a college degree - may account for half of all voters in Ohio’s primary, according to Ruy Teixeira, a senior fellow at the Center for American Progress in Washington.