Chicago — THE integrity of research and expert opinions in Washington came into question last week, prompting the resignation of Robert Litan, an economist, from his position as a nonresident fellow at the Brookings Institution.

Senator Elizabeth Warren raised the issue of a conflict of interest in Mr. Litan’s testimony before a Senate committee examining a proposed Labor Department rule designed to protect consumers in their dealing with retirement-plan brokers.

The testimony was based on a paper Mr. Litan had prepared for the Capital Group, a mutual fund company. Mr. Litan disclosed that the Capital Group, which has a stake in the debate, had funded his paper, but he did not disclose that it had also commissioned it. Mr. Litan concluded that the regulatory rule, while well intentioned, would be too costly. He resigned because he testified as a Brookings fellow, violating a recent Brookings rule change that would have prohibited that.

Senator Warren was herself criticized by economists and pundits, on the left and right. Hal Singer, a fellow at the Progressive Policy Institute and a co-author of the research she criticized, said, “This is McCarthyism of the left.”