Republican congressional leaders are charging that the Environmental Protection Agency colluded with environmental lobbyists to develop a plan, unveiled earlier this month, for the most sweeping -- and perhaps unconstitutional -- changes ever undertaken in existing U.S. power generation.

The charges of collusion have been vigorously denied by the EPA and the top echelons of the National Resources Defense Council (NRDC), an environmentalist mega-lobby whose officials have also, on occasion, been EPA office-holders, along with other environmental groups.

Nonetheless, Sen. James Inhofe, chairman of the Senate Environment and Public Works Committee, continues to assert that the Clean Power Plan was "years in the making behind closed doors," and allowed "special interests to set an agency's agenda while excluding states and other interested parties from the process."

Late last week Inhofe's charges took on new immediacy after 15 U.S. states ranging from West Virginia and Florida to Ohio and Michigan filed suit against the EPA's fast-track scheduling for the plan, which demands that states carry out an average 32 percent reduction in carbon emissions from existing electrical power generation facilities by 2030.

The EPA's Aug. 3 "final rule" on the reductions, which the agency has touted for its "flexibility," gives all U.S. states just 13 months to come up with initial proposals on how they intend to carry out the objective -- or federal agency could impose a plan for them. Only if EPA approves would they get another two years to produce a detailed version of their plan.

The protesting states petitioned the D.C. Circuit of the U.S. Court of Appeals for suspension of all EPA's deadlines, calling the new rule "the most far-reaching energy regulation in this nation's history" and "clearly unlawful."

CLICK HERE FOR THEIR PETITION

They argue that the EPA timetable for states to cut back on carbon-based fuels and "fundamentally reorganize their energy grids" is impossible to achieve, would force them to discard billions in on-going energy efficiency investments, and short-circuits the kind of careful study required for such drastic changes.

The EPA's hurry-up timetable is far less than the amount of time that EPA itself took to come up with the Clean Power Plan. That span was allotted under a controversial "sue and settle" agreement with a variety of environmental lobbying organizations, including NRDC, and a grouping of liberal states that was initially signed in 2010 -- and subsequently amended by the collaborating parties as the agency grappled with different approaches to the carbon emissions issue.

The frequent give-and-take between the partners to such a voluntary agreement -- which take place outside the normal process of public comment on agency rule-making-is one of the major reasons why the sue-and-settle process has been widely criticized by EPA critics and notably by Inhofe, as giving the agreement insiders a special priority.

Moreover, the approach that EPA took -- of setting broad "emission performance" goals and letting individual states find ways to meet them -- bears strong resemblance to a framework and strategy first proposed by NRDC in December 2012.

NRDC at the time hailed its notion as groundbreaking that EPA "set state-specific performance standards for existing power plants using national average emission rate benchmarks and the state-specific [power-producing] mix ... to produce state average fossil fuel emission rate standards" and subsequently went into 90 pages of detail on how to do it.

EPA's Aug. 3 final rule used that NRDC approach, while tinkering with among other things the various goals and time-periods of achievement

In February 2014, the New York Times seemed to agree with the notion of NRDC's influence. It declared that EPA "is looking closely at a proposal by the Natural Resources Defense Council, a nonprofit group, that could well be the heart of the regulation: states could comply with the rule not just by cleaning or shutting down coal plants, but also by making far broader changes across the electricity system-reducing demand, investing in 'smart grid' technology or supporting more renewable sources of energy."

Five months later, in July 2014, the Times declared flatly that a proposed version of EPA's new rule "used as its blueprint" the work of three top NRDC officials and lauded it as "a remarkable victory for the National Resources Defense Council." The Times quoted an expert at another environmentalist non-profit as saying "the NRDC proposal has its fingerprints throughout this, for sure."

EPA Administrator Gina McCarthy immediately called the July Times article "preposterous," and an unnamed EPA official told Politico the NRDC idea was "simply 'first out of the gate.'"

NRDC's then-President, Francis Beineke, was, at a minimum, more discreet. In noting the publication of the draft version of the EPA rule on June 2, 2014, she made no mention of her organization's contribution to the outcome, even though NRDC issued a glossy publication touting its proposal in March 2013, four months after initially suggesting it.

CLICK HERE FOR THE NRDC PROPOSAL

Earlier this month, Republican staff members of Inhofe's Environment and Public Works committee issued a 73-page report summarizing its attempts to document the alleged EPA-NRDC collusion that it said "rebuts the Obama Administration's narrative that NRDC did not have any special access to EPA policy makers and that NRDC had minimal input into EPA's development of greenhouse gas rules for power plants."

The report claimed to demonstrate how "spurious sue and settle tactics could not be successful without the complicity of agencies like EPA," and called into question "the accuracy and completeness of EPA's public statements to date about its regulatory plans.

It charged that "the litigation settlement provided the environmental activists significant leverage to drive the timing of EPA's rulemaking and to influence the scope of its policies."

The report also said it "raises further questions about the propriety of the close coordination between EPA officials and environmental activists, which often occurred during furtive phone calls, through private email exchanges, and meetings at coffee shops, local parks, and NRDC's office, all without adequate transparency."

Much of the evidence in the Senate staff report consists of fragmentary emails, letters and other messages between EPA officials and NRDC and other environmentalist figures, obtained through congressional request.

But the report also discloses frustrating lapses in the staffer's evidence trail, including such admissions as "EPA did not provide the [Environment] Committee with additional documents concerning the development or internal consideration of this original proposed settlement agreement."

Many of the messages on NRDC's side come from the authors of the environmentalist organization's ground-breaking proposal for EPA's new rule: David Doniger, currently director of the organization's Climate and Clean Air Program and longtime counsel to the head of EPA's clean air program during the Clinton Administration; Dave Hawkins, director of NRDC climate programs and a former head of the EPA's air, noise and radiation division during the Carter Administration; and Daniel Lashof, now working as CEO of NextGen Climate America Inc., a group founded by green activist-billionaire Tom Steyer. Lashof is still an NRDC senior fellow.

On the agency side, they include important figures such as Michael Goo, had of EPA's office of policy and a former NRDC registered lobbyist.

Among other things, the report documents that NRDC officials used Goo's personal Yahoo account to send computer modeling data to the EPA officials, and noted that NRDC, not EPA, provided the document revealing the exchange to Senate committee investigators.

Such use of personal emails occurred on "several occasions," the report said, rather than through "outside channels that ordinarily would result in transparency through Congressional oversight and public record requests.

(Federal law was not changed until 2013 to make it explicit that government officials officially had to made sure such off-line correspondence was transferred to the public record.)

Committee staffers also noted such unusual get-togethers as a June 27, 2011 meeting between Gina McCarthy -- then head of EPA's Air and Radiation section, which regulates greenhouse gases -- and NRDC's Doniger at a Starbucks near EPA headquarters, during a time when several other meetings with NRDC took place officially at the agency offices.

However, the subject of the meeting was not revealed, and staffer attempts to match notices of meetings on NRDC's records with those of the EPA led to frustrating failure.

As the report notes, even though EPA officials since 2009 had been directed to make their working appointment calendars available to the public, "EPA's transparency falls short of what [was] pledged."

Among other thing's McCarthy's publicly available schedule "dates back only to July 19, 2013, the day after McCarthy was confirmed by the Senate to the Administrator position, rather than to June 2, 2009, when she was confirmed as Assistant Administrator for Air and Radiation."

The date after that Starbucks meeting, however, the report says NRDF's Lashof sent a copy of his organization's "Climate Advocacy Strategy 2011-2012" to EPA's Michael Goo, once again using Goo's personal email.

Among other things, according to the staff report, that document declared that "the President has it within his power to make reductions to the two biggest sources of global warming pollution: power plants and cars...All he has to do is adopt rules that continue the recent rate of progress in these two sectors."

Such tid-bits, however, fall considerably short of proof of actual collusion.

Among other things, the documentation reveals that as EPA's intentions about how to achieve its clean-air goals mutated, so did the settlement agreement between the agency and the environmentalists, thus mandating continuing close communication between the two sides. Changes in the agreement, moreover, did not have to be made public, as the Clean Air Act did not require it.

Emails in the report also make clear that EPA was aware of NRDC's ground-breaking new proposals on how to regulate power plants before they were made public.

One email, dated Nov. 12, 2012, from EPA's then No. 2, Bob Perciasepe, tells Air Administrator McCarthy and others that the non-profit's report "about how to create flexible state implementation" was slated to come out "in several weeks."

In June 2013, EPA got a new spur forward when President Obama unveiled a Climate Action Plan designed to spur anti-greenhouse gas activity across the Administration. According to the report this led to new communications with NRDC.

For instance, in late June 2013, the report relates that [NDRC's] Lashof sent a "lengthy message" to a key EPA official discussing how EPA could develop greenhouse gas suppression plans for states "in a way that could avoid potential legal problems stemming from a separate Clean Air Act case pending before the Supreme Court."

These were followed up with other phone calls and meetings "to discuss technical analysis and modeling that NRDC had developed to support its policy proposals."

The committee majority staff report notes a spate of other meetings between the NRDC authors of the non-profit's state-centered regulated plan, especially as EPA's own plans began to mutate in a similar direction.

NRDC's Doniger, the report claims, "had full access to the EPA and documents obtained by the [Senate] Committee demonstrate that even EPA officials noticed him maneuvering from one official or office to another."

But some of the suggested intensity of those maneuvers seem less than overwhelming. According to the report, "EPA officials met or spoke by phone with Doniger and others from NRDC at least six times in the first four months of 2014, not including email exchanges." That is, about 1.5 times per month.

The Senate staff report argues, however, that the evidence shows that "groups such as NRDC were basically an extension of the Agency, providing policy advice, data and modeling, legal counsel, and even talking points."

NRDC, for one, see that differently.

"We are doing nothing more than petitioning our government -- a constitutionally protected right," declared Ed Chen, the organization's federal communications director. "That's our job. The real wrong here is for anyone to suggest we don't have the right to do so."

EPA pushed back just as strongly

"This is a flawed narrative driven by cherry picked and isolated communications that in no way reflect the full breadth and depth of the unprecedented outreach EPA engaged in to formulate and develop the Clean Power Plan," an EPA spokesperson told Fox News.

"There is simply zero merit to the idea that one group had any undue influence on the proposal's development," the spokesperson added.

"The final rule relies simply on the basic actions that states and utilities have already been taking to reduce CO2 emissions as we learned from our outreach process --and now this final rule responds to changes that stakeholders and states asked us to make."

But as last week's 15-state lawsuit against implementation of that EPA rule sharply makes clear, far from all of the states asked for any such thing.

And the debate over whose wishes got primary consideration -- and how -- is far from over.

George Russell is editor-at-large of Fox News and can be found on Twitter: @GeorgeRussell or on Facebook.com/GeorgeRussell