WASHINGTON — Since Carl Icahn, the billionaire investor, was named by President Trump as a special adviser on regulatory matters, he has been busy working behind the scenes to try to revamp an obscure Environmental Protection Agency rule that governs the way corn-based ethanol is mixed into gasoline nationwide.

It is a campaign that fits into the charge Mr. Trump gave Mr. Icahn, to help the nation “break free of excessive regulation.” But there is an additional detail that is raising eyebrows in Washington: Mr. Icahn is a majority investor in CVR Energy, an oil refiner based in Sugar Land, Tex., that would have saved $205.9 million last year had the regulatory fix he is pushing been in place.

Mr. Icahn, known internationally for his pugnacious and persistent approach to activist investing, has brought that same technique to his new role. He quizzed Scott Pruitt, a former Oklahoma attorney general, about the ethanol rule when Mr. Icahn helped interview Mr. Pruitt for the E.P.A. job. Mr. Icahn later reached out to Gary D. Cohn, Mr. Trump’s top economic adviser, to raise the issue. Mr. Icahn said he even had a telephone conversation in February with Mr. Trump himself.

The blitz has already generated at least one clear outcome: Since Mr. Trump was elected president with Mr. Icahn’s very vocal support and nearly $200,000 in political contributions to Republican causes — the stock price of CVR Energy has soared. By late December, it had doubled. It is still up 50 percent from the pre-election level, generating a windfall, at least on paper, of $455 million as of Friday.