“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. Brookings’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.

The shift has occurred as nonprofits in general have been under increasing pressure from their donors to meet specific goals. But for think tanks, that pressure can threaten their standing as independent arbiters in policy debates in Congress, the White House and the news media.

“Wouldn’t it be nice to go back to the greatest generation, in the post-World War II era of philanthropy, where they said, gosh, ‘Here is $1 million; spend it how you wish,’” Kimberly Churches, the managing director at Brookings, said in an interview.

Think tank executives reject any suggestion that they are tools of corporate influence campaigns and say they are simply teaming up with donors that have similar goals, like helping cities with economic development.