Despite the casual punditry, it's misleading to talk of Rick Santorum "sweeping the South." He's sweeping evangelicals and rural voters. There happen to be more of those in the South and Great Plains than elsewhere, and so Santorum does well. But even in the South, Romney wins in population centers, and among more educated and affluent voters. Romney carried the 3 counties around Jackson, Mississippi, with a plurality of 39%—better than Santorum's plurality statewide.
If Santorum cannot win Jackson in a Republican primary, how does he fare in Raleigh-Durham in the general? Tonight's results tell me: If Santorum were the nominee, President Obama will win North Carolina.