At least 18 major companies—and perhaps nearly twice that many—have ended their memberships in two of the nation's top coal lobbying groups, the National Mining Association and the American Coalition for Clean Coal Electricity since 2009, according to research by the Climate Investigations Center (CIC), an environmental advocacy group.

Volvo Construction Equipment North America publicly announced its departure from the NMA during the Paris climate talks last December, citing opposition to the group's anti-climate action lobbying. "We do not share the NMA's view on climate change nor their opinion about the politics on climate change driven by American policy," it told the environmental group Greenpeace. SFK, a Swedish manufacturer, made a similar public pronouncement.

But most of the companies—a list that includes utilities and banks as well as oil, coal and insurance companies—have exited quietly. To track those departures, the Climate Investigations Center monitored the groups' membership lists, contacting more than 20 of the major companies whose names had been removed to confirm their departures and reasons for leaving. Only some of those companies responded. Survey responses were published.

Some companies said they left because of the groups' positions on climate change policy, but others said it was part of a routine assessment of memberships and costs or budget constraints. Some utilities cited their transition away from coal as the reason for cutting ties. Others gave no reason for leaving.

The lobbying groups represent America's struggling coal producers and coal-burning utilities and have been involved in the fight to dismantle the Clean Power Plan, the Obama administration's signature climate policy that would limit carbon dioxide emissions from coal plants. It is currently under a stay by the Supreme Court.

The coal industry has been reeling in recent years due to federal pollution regulations and competitive pressure from natural gas and renewable energy. About 200 coal plants have closed, and many more are on the brink. Leading coal giants Arch Coal, Alpha Natural Resources and Patriot Coal filed for bankruptcy and coal production has plummeted to its lowest level in more than three decades. The industry's anti-climate action stance has also become a liability as corporate and public support grows for global climate action.

According to the report, nine companies confirmed leaving ACCCE and seven confirmed departing NMA. ACCCE lost Alcoa, Alstom, Ameren, DTE Energy, Duke Energy, E.On, FirstEnergy, GE Mining and Progress Energy. Five of these companies contributed between $1 million and $2 million to ACCCE when the group was first created in 2008. Meanwhile, NMA lost Anglo American, Chevron, Pacificorp, SKP, Volvo, Wells Fargo and Zurich.

Report author Joe Smyth, a CIC researcher, said: "If a lobby group loses a member here or there, okay, whatever. But if a lobby group loses a major utility that gave a million dollars [or more]...to start the group, I think that's quite significant."

CIC could not confirm the departure of 10 other companies from ACCCE and five others from NMA whose names were removed from the website membership lists. Since the CIC report was published last Tuesday, two more companies, coal firms Consol Energy and Alpha Natural Resources, confirmed leaving ACCCE.

"As with any coalition, membership and dues fluctuate over time, as do priorities," Laura Sheehan, a spokeswoman for ACCCE, said in an email to InsideClimate News. "In this instance, we've lost some members due to market realities which have caused major restructuring and even bankruptcies."

ACCCE currently has 32 members, including Peabody Energy and Alpha Natural Resources. NMA has more than 200 members.

With the exception of Volvo and SFK, "companies we know of left for economic reasons alone. Not surprisingly," said NMA spokesman Luke Popovich. "Every trade association representing cyclical industries will lose members for economic reasons in a prolonged downturn."

The groups also saw a decline in revenue and lobbying spending, according to the report. Citing available tax filings, the report said ACCCE's "lobbying and political spending" in 2014 was just $1.8 million, compared to $11.9 million in 2011. It also found the group's revenue was nearly $19.5 million in 2014, down from its peak of about $54 million in 2009.

The departures come as several high-profile companies, including Google, Facebook and Shell, have severed ties with the lobbying group the American Legislative Exchange Council (ALEC) because of its anti-climate lobbying.

"I would like to view this as some kind of wave of [climate] enlightenment taking over corporate America," said WildEarth Guardians climate campaigner Jeremy Nichols, who was not involved in this report, about the coal group departures. But it's "probably a cost-benefit analysis for these companies."

The shrinking membership and budgets of the lobbying groups provides more evidence of the coal industry's decline, said Tom Sanzillo, director of finance at the Institute for Energy Economics and Financial Analysis. Sanzillo did not contribute to the CIC report.