James Carter IV, 35, (the former president is James Earl Carter Jr.) told the Associated Press he searches YouTube every few days for keywords like "Romney" and "Republicans" in his role as a self-appointed opposition researcher.
One day in August, he found a clip of Romney, in an undisclosed location, discussing a visit to a Chinese factory where workers living in barracks-like dormitories work long hours for little pay in harsh conditions.
"The hidden camera video -- it was all blurred out at the beginning, and it was mysterious," Carter told the AP. "It piqued my interest."
Eventually, after sharing links to the video on Twitter, Carter found he had a new follower with the same name as the YouTube account. He quickly shot off a direct message.
"They were wary at first," Carter said. "But they did respond."
With some effort, he convinced the source -- whose identity still has not been revealed -- that he could be trusted.
Carter showed the source evidence that he had helped Corn report a story about Global-Tech Appliances Inc., a Chinese firm that Romney's Bain Capital briefly invested in. Both Carter and the source suspected it was that firm's factory that Romney was speaking about in the video.
"That gave me credibility," Carter said.
Then Carter persuaded the source to trust Corn with the full video on the condition that he keep the source's identity a secret.
"Carter was looking at this clip that came out, and said let's find this guy," Corn said. "So he started digging, and made contact with the source, and without knowing what the source had, he then put me in touch with him."
As for the source who taped Romney, Corn said, "what I'm told is that this was not a hit job or an infiltration, he didn't go there looking to get Romney. He just thought it would be kind of interesting."
On Monday afternoon, Corn posted some of the clips to his magazine's website, sparking a firestorm for the Romney campaign over remarks claiming that nearly half of Americans "believe they are victims" deserving government help and that the Palestinians have no interest in peace with Israel.
He released the entire video of nearly 50 minutes Tuesday, which you can see and hear below.
"James: This is extraordinary. Congratulations! Papa," Jimmy Carter told his grandson Tuesday morning in an email obtained by the AP.
"I've gotten a lot of Twitter messages from people supporting me and saying that it's poetic justice that it was a Carter that uncovered this, considering the way that the Romney campaign has been talking about my grandfather," Carter told the AP. "I agree with that sentiment wholeheartedly."
He also said he has gotten a string of job offers Tuesday, from the Ohio Democratic Party to liberal online news sites like The Huffington Post and ThinkProgress.
Romney video part 1, from Mother Jones:
Romney video part 2, from Mother Jones:
David Corn of Mother Jones tells Howard Kurtz of the Daily Beast how he got the Romney video: