Republican presidential contender Herman Cain has a provocative plan for reforming the US tax code: No federal tax higher than 9 percent for anyone.His “9-9-9” tax plan has resonated with many Republican voters for its simplicity and boldness. It’s one factor that has propelled Mr. Cain upward in recent weeks as other presidential hopefuls have been faltering.

But now, with many people looking closer at Cain as a top rival to former Massachusetts Gov. Mitt Romney for the Republican nomination, the 9-9-9 tax plan is also drawing closer scrutiny. Perhaps the most politically sensitive question concerns fairness.