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.
At the level of sober, fact-based conversation, that's an almost ridiculously obvious distinction to make. Of course, politics takes place at a very different level, and a lot of the public debate around tax rates and reform gets its oxygen from the slippage between the two points: "they only paid x percent" can quickly take on the emotional weight of "they're cheaters." But they aren't. We just need better laws.