“It’s shocking and insulting to be told before you go in to alter your testimony to what the administration wants,” he said. “This just shows a certain amount of amateurishness about how these hearings work. They’re supposed to be places where you get objective views. You don’t go around telling people what to say.”

Mr. Jackson and the E.P.A. communications office did not respond to emailed questions about Mr. Jackson’s communications with Dr. Swackhamer.

Representative Lamar Smith of Texas, the chairman of the committee, dismissed the accusations.

“It’s disappointing that the minority is politicizing what seems to be nothing more than a federal agency making sure that information provided to Congress is accurate,” he said in a statement. “Dr. Swackhamer and the Minority have repeatedly stated that she was testifying in her personal capacity and not in connection with her role as chair of the E.P.A. Board of Scientific Counselors. However, it is clear that the Minority invited her in an attempt to hijack the stated purpose of the Committee’s hearing on states’ role in E.P.A. rulemaking and shift the focus to recent E.P.A. actions involving the” Board of Scientific Counselors.

“Any attempt by E.P.A. to ensure that what Congress heard in testimony about official E.P.A. matters included the full breadth of information seems entirely appropriate,” Mr. Smith continued. “Unfortunately, the Minority has made the choice to waste taxpayer dollars as part of a politically motivated agenda.”

Dr. Swackhamer said she had already submitted her testimony to the congressional committee by the time she received Mr. Jackson’s email. She had also told the Science Committee and the E.P.A. that she planned to speak in her role as an independent scientist rather than an E.P.A. employee, and that she would make this plain at the outset of her testimony.

Dr. Swackhamer made her emails with Mr. Jackson available to the committee’s Democrats. On Monday, the panel’s Democrats, led by the ranking member, Eddie Bernice Johnson of Texas, sent a letter to Mr. Pruitt expressing concern that Mr. Jackson’s attempt to shape Dr. Swackhamer’s testimony may have been improper or even illegal. The Democrats requested that the agency’s inspector general investigate the matter.

“We contend that Mr. Jackson, and perhaps other senior E.P.A. employees, attempted to interfere with the testimony of an independent scientist to the Science Committee and may have sought to mislead Congress,” they wrote.