New friendships grew old, then cold. Environmental groups that once took money from McClendon — or considered doing so — to make a common cause against coal power, have stepped back as they weigh the environmental perils of extracting natural gas from shale, a business in which McClendon’s Chesapeake Energy is a leader.

The Sierra Club took $26.1 million in contributions from McClendon and Chesapeake-affiliated companies between 2007 and 2010, a fact that its executive director, Michael Brune, first disclosed to Time magazine earlier this month. Last year, Brune walked away from Chesapeake and an offer of an additional $30 million in donations.

“Identifying a common area of interest is very different from having our financial health dependent on a particular industry, or particular company,” he said in an interview.

The American Lung Association also has accepted an undisclosed amount from the company since 2009 for its “Fighting for Air” branding campaign. The Natural Resources Defense Council (NRDC), whose leaders McClendon wooed and who toured field operations, ultimately declined the funding he offered.

Brune decided that the donations represented a political liability when he took the Sierra Club’s helm in 2010. He said he realized that hydraulic fracturing, or “fracking,” was “going to be controversial, and I thought the most important thing was to strengthen our natural gas campaign.” He also faced complaints from local Sierra Club chapters in New York state and Pennsylvania.

It made sense that environmentalists viewed the natural gas industry as an ally when they were trying to forge a climate deal on Capitol Hill in 2009 and 2010, said Deborah Gordon, a senior associate in the Carnegie Endowment for International Peace’s energy and climate program.

“When cap-and-trade was going through, they needed an alternative. . . . They saw it as the savior, and it’s anti-coal,” Gordon said. But now, she said, concerns about the chemicals used to tap shale gas have become more pressing as hydraulic fracturing activity has increased nationwide. “It’s just exploded,” she said.

Although McClendon may be the gas industry’s most generous donor to environmental causes, he is not the only one. Natural gas entrepreneur T. Boone Pickens gave $453,250 to the liberal think tank Center for American Progress (CAP) in 2008 and 2009 through his nonprofit groups, to support its National Clean Energy Project events. At the time, Pickens was pressing lawmakers to adopt a bill to subsidize construction of natural gas filling stations. The legislation would have directly helped a company Pickens co-founded called Clean Energy Fuels, which describes itself as “the leading provider of natural gas for transportation.”