Some legal scholars, including some who normally lean to the left, believe the states have identified the law's weak spot and devised a credible theory for eviscerating it. The power of their argument lies in questioning whether Congress can regulate inactivity -- in this case by levying a tax penalty on those who do not obtain health insurance. If so, they ask, what would theoretically prevent the government from mandating all manner of acts in the national interest, say regular exercise or buying an American car?