This article first appeared in the Nation. For more great content, subscribe to the Nation here.

The Center for American Progress, Washington’s leading liberal think tank, has been a big backer of the Energy Department’s $25 billion loan guarantee program for renewable energy projects. CAP has specifically praised First Solar, a firm that received $3.73 billion under the program, and its Antelope Valley project in California.

Last year, when First Solar was taking a beating from congressional Republicans and in the press over job layoffs and alleged political cronyism, CAP’s Richard Caperton praised Antelope Valley in his testimony to the House Committee on Energy and Commerce, saying it headed up his list of “innovative projects” receiving loan guarantees. Earlier, Caperton and Steve Spinner—â€¨a top Obama fundraiser who left his job at the Energy Department monitoring the issuance of loan guarantees and became a CAP senior fellow—had written an article cross-posted on CAP’s website and its Think Progress blog, stating that Antelope Valley represented “the cutting edge of the clean energy economy.”

Though the think tank didn’t disclose it, First Solar belonged to CAP’s Business Alliance, a secret group of corporate donors, according to internal lists obtained by The Nation. Meanwhile, JosÃ© Villarreal—a consultant at the power-â€¨house law and lobbying firm Akin Gump, who “provides strategic counseling on a range of legal and policy issues” for â€¨corporations—was on First Solar’s board until April 2012 while also sitting on the board of CAP, where he remains a member, according to the group’s latest tax filing.

CAP is a strong proponent of alternative energy, so there’s no reason to doubt the sincerity of its advocacy. But the fact that CAP has received financial support from First Solar while touting its virtues to Washington policy-makers points to a conflict of interest that, critics argue, ought to be disclosed to the public. CAP’s promotion of the company’s interests has supplemented First Solar’s aggressive Washington lobbying efforts, on which it spent more than $800,000 during 2011 and 2012.

“The only thing more damaging than disclosing your donors and having questions raised about the independence of your work is not disclosing them and have the information come to light and undermine your work,” says Sheila Krumholz, executive director of the Center for Responsive Politics. “The best practice, whether required by the IRS or not, is to disclose contributions.”

Nowadays, many Washington think tanks effectively serve as unregistered lobbyists for corporate donors, and companies strategically contribute to them just as they hire a PR or lobby shop or make campaign donations. And unlike lobbyists and elected officials, think tanks are not subject to financial disclosure requirements, so they reveal their donors only if they choose to. That makes it impossible for the public and lawmakers to know if a think tank is putting out an impartial study or one that’s been shaped by a donor’s political agenda. “If you’re a lobbyist, whatever you say is heavily discounted,” says Kathleen Clark, a law professor at Washington University and an expert on political ethics. “If a think tank is saying it, it obviously sounds a lot better. Maybe think tanks aren’t aware of how useful that makes them to private interests. On the other hand, maybe it’s part of their revenue model.”

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When Newt Gingrich was running for president, The Washington Post ran a story about the Center for Health Transformation, which it described as his “hybrid” single-issue think tank. The center, which subsequently went bankrupt and was bought by WellStar, published reports and advocated on behalf of donors—including lobbyists and industry groups that donated millions to support its work—in addition to offering perks like “direct Newt interaction.” While the center did disclose some of its donors, it didn’t reveal how much money they had contributed.

It was an interesting story, but it obscured a key point: Newt’s “hybrid” was a particularly straightforward form of pay-to-play, but its basic features are common at Washington think tanks. Like Newt’s Center for Health Transformation, many lure big donors with a package of benefits, including personalized policy briefings, the right to directly underwrite and shape research projects, and general support for the donor’s political needs.

Most think tanks are nonprofit organizations, so a donor can even get a nice tax break for contributing. But it’s their reputation for impartiality and their web of contacts that makes them especially useful as policy advocates. “Think tanks can always draw a big audience to your event, including government folks,” a Washington lobbyist who has worked with several told me. “And people generally don’t think they would twist anything, or wonder about where they get their money.”

While think tanks portray themselves as altruistic scholarly institutions, they emphasize their political influence when courting donors. “If you have a particular area of policy interest, you can support a specific research effort under way,” the Brookings Institution says in one pitch for cash. Those interested in â€¨”a deeper engagement”—read: ready to fork over especially large sums of money—get personal briefings from resident experts and can work directly with senior Brookings officials to draw up a research agenda that will “maximize impact on policymaking.”

The Center for Strategic and International Studies advertises itself as being “in the unique position to bring together leaders of both the public and private sectors in small, often off-the-record meetings to build consensus around important policy issues.” It allows top-tier donors to directly sponsor reports, events and speaker series.

Because most think tanks don’t fully disclose their donors, it’s not always easy to see what sort of benefits money can buy. But during Chuck Hagel’s confirmation hearings, the Atlantic Council, where he’d been chairman before moving to the Pentagon, released a list of its foreign donors. One of them turned out to be the oil-rich government of Kazakhstan, headed by dictator Nursultan Nazarbayev. Last year, the council hosted a conference on Kazakhstan that was paid for by the Nazarbayev regime and Chevron, which has vast oil interests in the country and is also a major donor to the â€¨council. Keynote speakers included Kazakhstan’s former ambassador to the United States and Kenneth Derr, a former Chevron CEO and now Kazakhstan’s honorary consul in San Francisco.

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John Podesta, former chief of staff to President Bill Clinton and the head of Obama’s first transition team, founded the Center for American Progress in 2003. Last year, Podesta stepped down as CAP’s president—he remains its chair and counselor—and was replaced by Neera Tanden, who served in both the Obama and Clinton administrations. Former Virginia Congressman Tom Perriello heads the CAP Action Fund, an advocacy unit, which operates out of the same offices and shares personnel.

CAP has emerged as perhaps the most influential of all think tanks during the Obama era, and there’s been a rapidly revolving door between it and the administration. CAP is also among the most secretive of all think tanks concerning its donors. Most major think tanks prepare an annual report containing at least some financial and donor information and make it available on their websites. According to CAP spokeswoman Andrea Purse, the center doesn’t even publish one.