An antitax advocacy group zinged Republican presidential candidate Rick Santorum’s tax plan, giving him a grade of “D+” grade and the dubious honor of proposing what “may be the worst idea of any of the Republican candidates.” ”The good news is Santorum has gotten more specific about his tax plan since last month when we gave him a D+,” economist William McBride wrote on Thursday. “The bad news is… he’s gotten more specific.” Mr. McBride said the biggest problem with Mr. Santorum’s proposal is the sharply different corporate tax rates he would establish. Mr. Santorum would halve the corporate tax...