One of Barack Obama's most potent campaign claims is that he'll cut taxes for no less than 95% of "working families." He's even promising to cut taxes enough that the government's tax share of GDP will be no more than 18.2% -- which is lower than it is today.

It's a clever pitch, because it lets him pose as a middle-class tax cutter while disguising that he's also proposing one of the largest tax increases ever on the other 5%. But how does he conjure this miracle, especially since more than a third of all Americans already...