One of the most viral presidential ads to date, with over a million hits on Youtube, has been unleashed in a host of battleground states. And according to ABC News (and even Republican strategists), it may be the one Romney's campaign should fear most.

Here's ABC News' Matt Negrin:

The one-minute spot, made by the super PAC supporting President Obama, Priorities USA Action, already has more than 1 million hits on YouTube, more than 10 times any other super PAC ad. More significant, a person at a Republican group that has tested Democratic attacks on Romney told ABC News the ad is potentially the most devastating yet.

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The ad stars a former plant worker in Indiana who tells the story: One day he was told to build a small stage and, shortly after, Romney’s Bain colleagues herded the workers in front of the stage to tell them they were fired. Romney, the worker says, made $100 million by shutting down the plant.