Mitt Romney survived a relentless barrage of attack ads about Bain Capital during the Republican primary, but now there's one ad that appears to be breaking through more than any other.

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.

Posted June 23, it's running on TVs in Ohio and other battleground states, and it blames Romney for firing workers while making millions.

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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.

"I looked both ways. I looked at the crowd, and we all just lost our jobs," says the worker, Mike Earnest. "Turns out that when we built that stage it was like building my own coffin, and it just made me sick."

The Romney campaign has not responded to a request for comment.

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