Hmmm.. good point. I suppose it's easier to sell when setting up a system that treats multiple groups differently (each bracket), rather than singling out one single group. Also, demonizing one class ("they're not paying their fair share!") furthers the notion of class warfare. Saying, "we're in trouble as a country, we're asking those at the top to shoulder an even bigger burden than they already do" is a very different tone than "they're not paying their fair share!", which is in essence saying that "the rich" are ripping off everyone in every other class.
It's a red herring though, class warfare or not. It's not going to do anything meaningful toward covering the deficit, unless you truly jack up the upper rates by a large amount, which will never fly anyway.