And at Brookings, executives imposed new rules in December requiring that unpaid nonresident scholars use their title from any paying job, not from Brookings, if they testify before Congress. They are also prohibited from using their affiliation with Brookings in any research report they publish under contract with an outside party.

“These are designed to avoid not just conflicts of interests, but the appearance of conflicts of interests,” said Martin S. Indyk, the executive vice president at Brookings.

Separately, Brookings terminated its relationship with Mr. Litan in September after he failed to make clear that testimony he provided to a Senate committee last year, based on a study he had done as a private consultant, “left the false impression that Brookings was connected with the report and the testimony,” Strobe Talbott, the Brookings president, said in a letter explaining the matter.

Mr. Eisenach, for reasons he would not specify, said he was no longer working on issues related to net neutrality — even though he had taken up the topic on behalf of the American Enterprise Institute as recently as January and published an industry-funded report through his consulting firm in February that discussed it.

“I’ve moved on to other issues,” Mr. Eisenach said.

Such steps are long overdue, said Thomas Medvetz, the author of the 2012 book “Think Tanks in America” and an associate professor of sociology at the University of California, San Diego.

“It has gotten to the point where everyone in Washington has their own expert,” Mr. Medvetz said. “It is yet another reflection of the tremendous influence of economic power in American politics — as with money, you can create your own vehicles of political influence.”

Still, not everyone is worried about the multiple roles played by think tank scholars.

Representative Greg Walden, Republican of Oregon, oversaw a House hearing on the F.C.C.’s net neutrality rule early last year. Among the evidence he submitted into the congressional record was a Wall Street Journal op-ed article co-written by Robert M. McDowell, a Hudson Institute scholar who also serves as a telecommunications industry lawyer at a firm retained by AT&T to lobby on net neutrality.

“Everyone’s got their point of view,” Mr. Walden said in an interview last year. “And some of them get paid to have that point of view.”