/ Obama supporters like this woman who showed up to cheer at a campaign event in Melbourne, Fla., on Sept. 9, may not realize how much personal data the organization collected, or what it's doing with it now.

President Barack Obama’s presidential campaign has turned over its most valuable asset — a massive computer database containing personal data on millions of American voters — to a new advocacy group created to advance the White House agenda on issues ranging from gun control to immigration reform.

Organizing For Action (OFA), the advocacy group set up in recent weeks by the president’s top political aides, has already acquired access to the database under a leasing agreement with the Obama campaign, Katie Hogan, a former Obama campaign aide who is now serving as spokeswoman for the lobbying group, told NBC News. The information will be used to unleash an “army of the door knockers” to back the president’s legislative agenda as well as raise money for “issue ads” – particularly in crucial congressional districts, she said.

As an opening salvo, the group on Friday urged the president’s supporters to call members of Congress in support of Obama’s gun control proposals, even offering a sample script of what they should say.

The creation of OFA, which is being chaired by former Obama campaign manager Jim Messina, is stirring controversy – both among public interest groups over the group’s plans to accept unlimited corporate donations, and among privacy advocates over the transfer of the database.

“It’s extremely worrisome,” said Lillie Coney, associate director of the Electronic Privacy Information Center, noting that Obama campaign supporters likely have no idea that personal data they voluntarily shared with the campaign has now been transferred and is being used for purposes beyond the election.

Dubbed the “nuclear codes” by campaign aides, the Obama campaign database is widely described as one of the most powerful tools ever developed in American politics. According to published reports, it contains the names of at least 4 million Obama donors – as well as millions of others (the campaign has consistently refused to say how many) compiled from voter registration rolls and other public databases. In addition, the campaign used sophisticated computer programs — with code names like “Narwhal” — to collect information through social media: Anybody who contacted the campaign through Facebook had their friends and “likes” downloaded. If they contacted the campaign website through mobile apps, cellphone numbers and address books were downloaded. Computer “cookies” captured Web browsing and online spending habits.

“I can’t think of anything that rivals this data,” said Coney, noting that much of the data was voluntarily supplied by voters, something that consumers are often reluctant to do when dealing with commercial companies. “The private sector would love to be able to do what the (Obama) campaign was able to do.”

OFA spokeswoman Hogan said that Obama supporters have the option in emails they receive of opting out — or unsubscribing — from the list, as required by federal law. But critics say that is not necessarily an option for information collected about voters through other means (such as public databases) and note that many on the list likely don’t notice the “unsubscribe” fine print on the emails.

At the same time, OFA’s plans for corporate-backed lobbying of Congress have spurred sharp criticism from campaign reformers — a cause the president once championed. Fred Wertheimer, president of Democracy 21, a leading reform group, called OFA “dangerous and unprecedented,” noting that it has been set up under the same section of the tax code used by controversial GOP advocacy groups, such as Karl Rove’s Crossroads GPS (as a 501(c)(4) “social welfare” nonprofit organization). This will allow the group to accept unlimited donations from wealthy individuals and corporations.

“With his decision to allow corporations to fund the new organizations that will operate as an arm of his presidency, President Obama has ‘given a green light to a new stampede of special interest money,’” said Wertheimer in a statement that quoted Obama’s own words two years ago to denounce the Citizens United Supreme Court decision striking down many campaign finance limits. “This would take President Obama about as far away as he could possibly get from the goal he set in 2008 to change the way business is done in Washington.”

Related: Nonprofit spends big on politics despite IRS limitation

In response to a request for comment, a White House spokesman emailed recent comments by top Obama political adviser David Plouffe to ABC’s George Stephanopoulos: “Yes, we will voluntarily disclose all of our donors,” Plouffe said. “And we're very excited. The people who actually made the president's campaign in both '08 and '12, our great grassroots volunteers, were pretty clear after the election they wanted to stay with it and they want to be out there organizing, driving message, holding people accountable on issues like immigration, you know, the deficit and jobs, gun safety.”

But how much the group will disclose about the source of its money is still unclear. There is no legal requirement for a 501(c)(4) group like OFA to do so. Hogan, the OFA spokeswoman, declined to say how often the group will make disclosures or whether it will report amounts that donors give or simply provide a list of contributors. (Such a list -- without amounts detailed -- was recently released by the Presidential Inaugural Committee.) “That’s still being worked out,” she said.

As if to underscore the role of major corporations in helping to underwrite OFA, the unveiling of the group came at a special invitation-only event on inaugural weekend at the Newseum, sponsored by Business Forward, a corporate-backed trade group close to the White House, according to a Politico account. Business Forward -- whose charter members include Citi, Dow Chemical, Duke Energy, Ford, Google and Comcast, majority-owner of NBCUniversal, parent company of NBC News -- had lobbied for the White House-backed fiscal cliff deal, specifically touting its tax breaks for businesses, such as write-offs for new capital investment and research and development credits, according to a statement on the group’s website.

“We need you. This president needs you,” Messina said at the launch event, according to the Politico account, adding that the national advisory board of OFA will be “filled with people in this room.”

One corporate executive who attended the event told NBC News the roll out -- which featured a spirited talk by former President Bill Clinton on gun control -- drew numerous major Obama campaign bundlers and fundraisers, such as Obama campaign finance chairman Mathew Barzun (now reportedly a front-runner to be tapped for ambassador to the Court of St. James) and finance director Rufus Gifford.

“My takeaway from this was that they set this up to take advantage of the Citizens United decision and operate this outside the Democratic National Committee so they won’t have to file (election) reports,” said the executive, who asked not to be identified.

Hogan, the OFA spokeswoman, said that OFA will not run campaign ads — only “issue” ads that do not fall under the election laws.

But the underlying political purpose of the group is not disputed. “The way it’s organized, we legally can’t participate in elections,” Stephanie Cutter, a top Obama campaign official who now serves on the board of OFA, said at a recent Politico-sponsored inaugural event. “But that doesn’t mean the issues we’re organizing around won’t mobilize the American people to vote for things — to vote for that economy we’ve been working for, to vote for immigration reform, to vote for common sense gun reforms. I think we can affect elections, we just can’t legally be involved in them — for this particular organization.”

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