The eastern sky lightens just before sunrise over the Chesapeake Bay on the first morning of astronomical winter, Dec. 22, 2011, in North Beach. (Ray K. Saunders/Washington Post)

For more than a decade, the Potomac Riverkeeper organization has been that ant that tried to move a rubber tree plant, taking on large corporate polluters in high hopes of one day cleaning the Chesapeake Bay.

But recently, in an unlikely twist, the scrappy Potomac Riverkeeper backed away from a fight. Although it believes a controversial part of the Environmental Protection Agency’s plan to clean up the bay will actually lead to more pollution, it abandoned a small coalition of environmentalists that had threatened to sue the EPA. “We do not have the resources to get involved with this particular lawsuit,” said the group’s leader, Ed Merrifield.

It was responding to a threat — not from a corporation, its usual foe, but a friend. The Keith Campbell Foundation for the Environment, which has donated millions of dollars to Riverkeeper groups over the years, threatened to withdraw future funding if they sued. Other Riverkeeper groups also dropped legal action.

It was a clear sign that environmental groups have not only squared off against the American Farm Bureau Federation, which has filed a federal lawsuit to stop the largest bay cleanup plan in history, but also each other, in the 16 months since the EPA approved the first state proposals to reduce pollution in the watershed.

The stakes are huge. The most ardent backers of the EPA’s aggressive new pollution diet — the Choose Clean Water Coalition of 230 groups, the Chesapeake Bay Foundation and the Natural Resources Defense Council — view it as the Chesapeake’s last hope.

This 30-second time exposure of the Chesapeake Bay Bridge was made just before dawn Oct. 30, 2011. (Michael S. Williamson/WASHINGTON POST)

If it unravels because of a legal challenge, or lack of support from the states charged with implementing the cleanup, it might take 10 years to draft another plan to stop millions of tons of pollution from flowing into the bay, according to the bay foundation.

From the bay foundation’s perspective, a challenge to the EPA’s authority by the group’s allies is not much different from the challenge being heard in a Pennsylvania court by its foes — the farm, pesticide, pork and home-builder lobbies that stand against the strict regulations of the pollution diet.

Riverkeeper groups also want a clean bay, but some could not stomach one of EPA’s methods, a program that would turn the Chesapeake’s waters into an open marketplace for farmers and corporations that create pollution.

Under a nutrient trading program, farmers who exceed pollution reduction goals set by the EPA would receive credits they could sell to corporations such as coal-fired power plants that fail to reach their own reduction goals.

In theory, the program would help farmers pay for expensive crop covers and buffers to soak up rain. Storm runoff from farms is a major problem because it carries nitrogen from fertilizers and phosphorus from animal waste into streams and rivers that flow to the bay. The pollution contributes to oxygen-depleted “dead zones” that smother oysters, crabs, mussels and other marine life in the nation’s largest estuary.

But Riverkeeper group members and some other environmentalists say that nutrient trading is a shell game that will allow more pollution to creep into the bay. They say that because of lax farm regulations in bay watershed states — Virginia, Maryland, West Virginia, Pennsylvania, Delaware and New York — the EPA would have no sure way of knowing whether farmers have met pollution reduction goals.

The comprehensive plan is at a key stage. At the end of March, states submitted final proposals on how they would implement their strategies to reduce pollution in coming years.

Matt Rutherford, center, in his 27-foot Albin Vega sailboat, sails the last few miles up the Chesapeake Bay with a group of boats to accompany him in completing a solo, 314-day circumnavigation of the Americas, ending his voyage where he started, in Annapolis on April 21, 2012. (John McDonnell/The Washington Post)

With the threatened lawsuit, lawyers at two environmental groups, Food and Water Justice and Earthjustice, along with some Riverkeeper groups, sought to surgically remove nutrient trading from the comprehensive plan.

“People were very concerned that one of our partner organizations would sue” in a federal court, said Will Baker, president of the Chesapeake Bay Foundation.

The American Farm Bureau Federation, the National Association of Homebuilders and others challenged the EPA’s authority in a lawsuit last year, saying that the agency’s assessment of pollution is based on faulty science and that only states have the power to enforce a cleanup.

“We heard from a number of attorneys that when you have two pieces of federal litigation both supporting the same thing . . . they join together, and we thought that would have been terrible,” Baker said.

A solidarity meeting scheduled for March was abandoned, at least one environmentalist said, because people were screaming at a mediator. Not true, Baker said: “I have never been in any meeting about this topic where people have raised their voices.”

Shouting was not necessary, because money did the talking, said a member of a Riverkeeper group. Keith Campbell, founder of the Keith Campbell Foundation for the Environment, sits on the board of the Chesapeake Bay Foundation, which discussed the legal threat.

About two months ago, members of Riverkeeper groups gathered at the foundation in Annapolis for an important word from their sponsor. According to a member who attended the meeting, the foundation’s director, Verna Harrison, reminded them that the foundation had donated $4 million to their causes over the years and then issued an ultimatum.

“If you challenge nutrient trading, you’re done. You won’t be funded by us anymore,” said the Riverkeeper member, recalling Harrison’s words. The member declined to be named, fearing a funding cut. Merrifield of Potomac Riverkeeper also declined to comment on the dispute.

Harrison declined to comment on her discussions with Riverkeeper activists. In a statement, Campbell Foundation President Samantha Campbell said:

“When we find a fundamental disconnect between the principles that drive our mission and the work of a potential grant recipient, we take a number of steps to inform all parties on the priority areas that are currently being considered for funding.”

Scott Edwards, co-director of Food and Water Justice, which worked with Earthjustice to draft the lawsuit, said, “I’ve been at this for years and years, and I’ve never seen this attempt to try to stop a group for asserting its position.”

Without its partners, Food and Water Justice, an arm of Food and Water Watch, is trying, so far in vain, to revive the lawsuit. “You expect this from industry; you don’t expect it from your friends,” Edwards said.