“Entering into this arrangement causes a reasonable person to question the integrity of the E.P.A. decision,” said Don Fox, who served as general counsel of the Office of Government Ethics during parts of the Obama and George W. Bush administrations.

The March 2017 action by the E.P.A. on the pipeline project — in the form of a letter telling the State Department that the E.P.A. had no serious environmental objections — meant that the project, an expansion of the Alberta Clipper line, had cleared a significant hurdle. The expansion, a project of Enbridge Inc., a Calgary-based energy company, would allow hundreds of thousands more barrels of oil a day to flow through this pipeline to the United States from Canadian tar sands.

The signoff by the E.P.A. came even though the agency, at the end of the Obama administration, had moved to fine Enbridge $61 million in connection with a 2010 pipeline episode that sent hundreds of thousands of gallons of crude oil into the Kalamazoo River in Michigan and other waterways. The fine was the second-largest in the history of the Clean Water Act, behind the penalty imposed after the Deepwater Horizon spill in the Gulf of Mexico.

A spokesman for Williams & Jensen said that the lobbying firm did not intervene with the E.P.A. or Mr. Pruitt on the Enbridge pipeline expansion either before or after Mr. Pruitt was living in the condo owned in part by Vicki Hart, the wife of J. Steven Hart, the chairman of the firm.

The lobbying firm also said it had not worked on similar regulatory issues for Enbridge in the past year, even though it was registered at the time as lobbying for the company on “issues affecting pipelines and construction of new pipelines,” its disclosure report from early 2017 says.