The self-described Atlanta-based "oppo researcher" who helped broker the release of the secret video that has rocked the Romney campaign got a congratulatory email today from his famous grandfather -- former President Jimmy Carter.

GOP presidential nominee Mitt Romney defended his unguarded comments, secretly recorded at a private fundraising event in May and provided to the liberal magazine Mother Jones, that shows him speaking frankly about Obama's supporters. NBC's Michael Isikoff reports.

James Carter IV told NBC News in an interview that, starting late last month, he tracked down the source who took the secret Romney video via Twitter -- and then in a series of messages encouraged him to release the full tape to Mother Jones magazine.

After emailing his grandfather the magazine's story about the tape -- under the subject, "Huge campaign news," and calling it "my biggest story yet" -- the former president wrote back at 7:16 am Tuesday: "James: This is extraordinary. Congratulations! Papa."

"I'm proud of my role in being able to track him down," James Carter, 35, said about the source who took the video. "I'm a partisan Democrat. My motivation is to help Democrats get elected. If there is anything I can find in any race, I try to do that."

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But Carter also confirmed there is a personal side to the backstory of the campaign video: he was especially motivated, he said, because of Romney's frequent attacks on the presidency of his grandfather, including the GOP candidate's comparisons to the "weak" foreign policy of Carter and Barack Obama.

"It gets under my skin -- mostly the weakness on the foreign policy stuff," Carter said. "I just think it's ridiculous. I don’t like criticism of my family."

Carter said he is currently unemployed and has not been paid for his work by the Obama campaign or any other political organization. What motivated him at first was Romney's role at Bain Capital and the controversy over whether the GOP candidate as a businessman had invested in companies that outsourced jobs overseas.

Carter had focused, in particular, on Bain Capital's 1998 investment -- while Romney was still chief executive -- in Global Tech Appliances, a Chinese manufacturing company. Carter was listed as providing "research assistance" to a July 11 story about the investment by David Corn, Mother Jones' Washington bureau chief and an MSNBC contributor.

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Then, in late August, just before the Republican convention, Carter spotted a YouTube link to a brief video clip in which Romney talks about his investment in a Chinese company. The link was posted under the name "Rachel Maddow" but was quickly taken down because the poster had no relationship to the MSNBC host.

The video then reappeared on YouTube under a different account -- "Anne Onymous." Carter said he was fascinated by the video -- and figured there had to be more to Romney's talk.

"It was just weird video to all of a sudden come across,” he said. “It was all very strange and it piqued my curiosity," he said.

Carter Tweeted a link to the video -- and then soon noticed he had a new follower named "Anne Onymous."

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"I recognized it" -- and then messaged the follower back, resulting in a series of exchanges in which he encouraged the poster to come forward and give the full video to Corn.

The source who took the video has confirmed to NBC News that it was taken at a May 17 $50,000-a-plate fundraiser at the Boca Raton, Fla., home of private-equity mogul Marc Leder, chief executive of Sun Capital Advisors.

Leder has given $225,000 to Restore Our Future, the pro-Romney Super PAC, in addition to raising money for Romney's presidential campaign. He has also been the subject of controversy after a report in the New York Post last year -- under the headline "Nude Frolic in Tycoon's Pool" -- about a wild party at his Bridgehampton mansion in which, according to the Post's account, "guests cavorted nude in the pool" and scantily clad Russian dancers performed on platforms.

Leder has not responded to a request for comment from NBC News.