The definition of a low blow in this year's Republican presidential primary? Suggesting that your opponent might not -- gasp! -- truly love former President Ronald Reagan with 110 percent of his heart. That's what the campaign of Ariz. Sen. John McCain has just done to its rival, former Mass. Gov. Mitt Romney.

Romney has a history of somewhat malleable political positions; having run to the left in earlier elections, he's now positioning himself as the conservative candidate rising up to stop McCain, who is strongly disliked by certain segments of the conservative movement. McCain clearly isn't willing to take that line of attack lying down, as his campaign has just released the ad below. Called "Trust," the ad presents some of Romney's previous statements on Reagan and poses the question: "If we can't trust Mitt Romney on Ronald Reagan, how can we trust him to lead America?"