After years of secrecy, Barack Obama’s aides are finally dishing on the most powerful asset in his campaign arsenal.

Obama’s aides always refused to discuss the campaign’s massive email network, which shattered fundraising records and recruited the largest fleet of volunteers in the history of American politics. In 2004, John Kerry’s list hit three million, and some estimated Obama’s list could top five million. That would make it double the size of the largest email lists in U.S. politics, including older web groups like MoveOn. It turns out those estimates weren’t even close.

Obama’s list now tops a whopping ten million people, according to today’s Washington Post.

The article does not directly attribute that figure to anyone. The same paragraph cites "senior aides," however, to report that the list is so financially valuable that it was "briefly offered" as loan "collateral during a cash-flow crunch." A source in a position to know also told me that the email list has reached eleven million people.

So how did this information go from the Democratic Party’s best kept secret to an announcement in The Washington Post?

Because now, Obama’s team wants everyone to know. The massive list of energized activists is the biggest stick Obama will carry in Washington.

It enables direct communication at a remarkable scale. The next President can instantly address 16 percent of his national supporters, based on the popular vote. To put it another way, the list dwarfs the audience of all the nightly cable news shows combined.

So even after the gauzy honeymoon talk fades, when people start second-guessing how much "political capital" Obama really has, there will be this resilient network of people committed to enacting the Obama agenda. In a policy fight with Congress — or a message battle with the press — these are the people that will take action to get Obama’s back. They will call their neighbors, or their members of Congress. They’ll knock on doors, or storm local meetings. They’ll write letters to the editor or, naturally, email and prod their networks. They can also hold Obama accountable, of course, by using the same networked technology to pressure the new administration. Peter Daou, a web strategist and former adviser to Hillary Clinton’s campaign, raised that prospect in the article:

…Obama faced an intense backlash when he [changed his position on] the issue of immunity for telecommunications companies that took part in the warrantless wiretapping program. "People who have helped you reach this historic goal by self-organizing can also organize in opposition to your policies," [Daou] said.

Obama supporters converted his website into a protest hub against his FISA position last summer, a presidential campaign first that drew coverage from blogs, The Nation, and ultimately traditional media. They can swiftly organize again. I think it will be even easier now, because traditional journalists are ready to jump on these kind of stories, and media coverage is crucial to growing net movements. Activism focused on pushing Obama, however, is not likely to be an immediate priority. After all, supporters are energized by this victory, and there is a broad consensus on the short-term priorities of the economy and Iraq. (One current effort to rally online opposition to one of the more controversial names floated for Obama’s cabinet, Larry Summers, has hardly gained any traction.) Far beyond base activists, the new administration also has an opportunity to tap technology for a more open, transparent and interactive government.(I plan to explore that potential in an upcoming magazine article).

Obama’s email network is especially intriguing for governance, however, because it has the potential of acting as both his most powerful grassroots tool and the most visible check on a President at the helm of one-party government.