The Christian Century

So it turns out that the president and first lady's tax burden for last year was only 20.5 percent. Does this make Obama a hypocrite for criticizing Mitt Romney's low tax rate?

Only if he blames Romney personally for not voluntarily paying more. As I said in Romney's defense a while back, the problem isn't that presidential candidates with plenty of money aren't willing to pay their taxes. The problem is that their taxes are too low.