A waning crescent moon hangs over the Chesapeake Bay just before sunrise in North Beach, Md. Twenty one states are challenging the Obama administration’s plan to clean up the bay. (Ray K. Saunders/The Washington Post)

Attorneys general in 21 states are backing an attempt to derail the Obama administration’s Chesapeake Bay cleanup plan, fearing that the government will use that authority to regulate wastewater in other watersheds, including the Mississippi River Basin.

State attorneys general, most of them Republicans, from as far as Alaska and Montana joined the American Farm Bureau Federation in its fight to prevent the Environmental Protection Agency from carrying out its plan to clean up the nation’s largest estuary. Impaired waters have led to fish-killing dead zones and other marine life die-offs for decades.

“If this [cleanup] is left to stand,” they argued in their joint amicus brief filed this week in the U.S. Court of Appeals for the 3rd Circuit, “other watersheds, including the Mississippi River Basin, could be next.”

Although farm and industrial pollution in the Mississippi River causes an immense Gulf of Mexico dead zone that kills marine life, the EPA has said it has no interest in orchestrating a cleanup plan that states in the region haven’t requested and aren’t prepared to develop, unlike the Chesapeake Bay region.

The EPA recently challenged and lost a federal suit filed by environmental groups that called on it to end its “hands-off approach” to managing the Mississippi River and to develop a measuring stick for the level of pollution that could be allowed to enter the river.

In a statement about the brief, the attorneys general described it as a bipartisan agreement, although it includes only three Democrats. The 21 states were described as geographically varied, but they were solid in their opposition to President Obama in the 2012 election, with the president taking only Michigan and Florida.

The other states are Alabama, Arkansas, Georgia, Indiana, Kansas, Kentucky, Louisiana, Missouri, Nebraska, North Dakota, Oklahoma, South Carolina, South Dakota, Texas, Utah, West Virginia and Wyoming.

Their action drew a sharp response from the Chesapeake Bay Foundation, which sued the EPA to initiate the cleanup plan. “We say to . . . [the 21] states, don’t tell us how to restore clean water in our backyard,” said its president, Will Baker. “Together, we are well on our way to making our rivers and streams safer, improving habitat, protecting human health, and strengthening local economies.”

Obama issued an executive order to restore the Chesapeake’s health in May 2009. The next year, the EPA embarked on an aggressive program to limit the tremendous amount of waste that pours into the bay from overflowing municipal wastewater management systems, which are regulated under the Clean Water Act, and farms and animal feed operations, which are not.

The EPA stepped in to organize efforts by six bay states — Virginia, Maryland, West Virginia, Pennsylvania, Delaware and New York, as well as the District — when they failed for years to lower pollution on their own.

Maryland and the District were eager for a cleanup, but other states were reluctant. Using its powers under the act, the EPA threatened to withhold permits that would have limited construction projects and allowable sewer releases if the states did not come up with individual pollution-

reduction plans by a December 2010 deadline.

Within months of the agreement by the states and the federal government to tackle the problem, the farm bureau filed a suit in federal court in Harrisburg, Pa., in early 2011 to stop it. Groups such as the Fertilizer Institute, National Pork Producers Council and National Chicken Council joined the suit.

They said a “pollution diet” costing taxpayers and farmers billions by its full implementation in 2025, with upgrades to deteriorating sewer facilities and fences to limit chemical runoff from farms, is the sole responsibility of states. A judge dismissed their demand for an injunction in September and ruled that the agreement could move forward. The amicus brief was submitted in support of their appeal.

West Virginia was the only bay state to sign the amicus brief. Its Republican attorney general, Patrick Morrisey, took office last year. Democrat Darrell McGraw was in office at the time of the agreement.