Think tanks, which position themselves as universities without students," have power in government policy debates because they are seen as researchers independent of moneyed interests. But in the chase for funds, think tanks are pushing agendas important to corporate donors, at times blurring the line between researchers and lobbyists. And they are doing so while reaping the benefits of their tax-exempt status, sometimes without disclosing their connections to corporate interests. On issues as varied as military sales to foreign countries, international trade, highway management systems and real estate development, think tanks have frequently become vehicles for corporate influence and branding campaigns. "This is about giant corporations who figured out that by spending, hey, a few tens of millions of dollars, if they can influence outcomes here in Washington, they can make billions of dollars," said Senator Elizabeth Warren, Democrat of Massachusetts, a frequent critic of undisclosed Wall Street donations to think tanks. Washington has seen a proliferation of think tanks, particularly small institutions with narrow interests tied to specific industries. At the same time, the brand names of the field have experienced explosive growth. [The Brookings Institution]'s annual budget has doubled in the last decade, to $100 million. The American Enterprise Institute is spending at least $80 million on a new headquarters in Washington, not far from where the Center for Strategic and International Studies built a $100 million office tower.