Reacting to the email exchange, Mr. Vitter said in a statement: “Who is working for whom? The key example in all of this is the settlement agreement on greenhouse gases when the N.R.D.C. sued the E.P.A., the E.P.A. settled, and the two celebrate the agreement. It doesn’t get any more blatantly obvious than that.”

The emails were provided to The New York Times by Republican staff members on the Senate environment committee. Mr. Vitter and Mr. Issa said their investigation was prompted by an article in The Times in July that detailed how the Natural Resources Defense Council influenced key elements of the June climate change regulation.

E.P.A. officials said that the environmental group did not wield outsize influence in shaping the regulation, and pointed out that the agency sought comments from hundreds of groups, including environmental advocates, state regulators, electric utilities, labor groups, tribes and the coal industry, when preparing the rule.

“To imply that one group had any undue influence on the proposal’s development is ridiculous and absurd,” Thomas Reynolds, an E.P.A. spokesman, said in an email. Ed Chen, the council’s communications director, said the group was simply doing its job. “If we weren’t trying to motivate the E.P.A., or the White House, to protect public health, we’d be guilty of malpractice,” Mr. Chen said in an email.

While the council was suing the Obama administration in order to create the legal requirement for the climate change rule, three of the group’s environmental experts were also drafting a proposal for what the rule should look like. The three — Mr. Doniger, David Hawkins and Daniel Lashof — produced a 110-page plan, with the explicit aim of offering it to the Obama administration. It was widely viewed by experts as the blueprint for Mr. Obama’s climate change rule.

The men had been corresponding with Ms. McCarthy since she was named assistant administrator of the E.P.A. in the first months of the Obama administration. At one point Mr. Lashof, a climate scientist, asked her for a job.

“As the Obama administration seeks to make rapid progress on air pollution, climate and energy, I am uniquely positioned to help forge a workable and effective strategy that combines Clean Air Act implementation and new legislation,” Mr. Lashof wrote to Ms. McCarthy in March 2009.