WASHINGTON - A group of environmentalists, watermen and former politicians will announce today that it plans to sue the U.S. Environmental Protection Agency for botching the cleanup of the Chesapeake Bay. The suit will seek to accelerate a 25-year-old program that has left the famous estuary just as troubled.

The announcement - from the nonprofit Chesapeake Bay Foundation, the Maryland and Virginia watermen's associations, former District of Columbia mayor Anthony A. Williams (D) and several others - marks a change in the tone of environmentalism around the bay. Previously, the groups had tried to prod the slow-moving, deadline-missing government cleanup program with statehouse lobbying or sobering "State of the Bay" reports.

Now they are threatening to bypass it.

(Pennsylvania contributes significant pollution to the bay in the form of drainage from sewer and septic systems, and runoff from farm fields and parking lots, much of which drains into the Susquehanna River, and then into the bay.)

In a letter to be sent to the EPA, the advocates say they want a federal judge to force new deadlines on local, state and federal bureaucrats so that all the measures necessary for a healthy Chesapeake will be in place by 2015.

That would be five years after the current deadline for a clean bay, 2010, which EPA officials have conceded they will not meet.

"Sometimes you have to hit the mule in the head with a two-by-four to get his attention. That's what we're doing now," said former Maryland state senator C. Bernard "Bernie" Fowler, a longtime advocate for the bay and another member of the group that sent the letter. Given the decline in the Chesapeake's health, Fowler said, "we just didn't have another choice."

The bay, which Baltimore writer H.L. Mencken called an "immense protein factory" for its abundance of fish and shellfish, is a vast estuary troubled by microscopic contaminants. The pollutants nitrogen and phosphorus wash downstream from sewage plants, septic tanks and manure-laden farm fields, from western New York to Southside Virginia.

The pollutants are plant food, and in the Chesapeake they fuel unnaturally large blooms of algae, which suck up the oxygen that fish, crabs and other creatures need to live. The low-oxygen "dead zones" were the Chesapeake's main problem in 1983, when leaders from Pennsylvania, Maryland, Virginia and the EPA signed the first agreement promising to help the bay come back.

They remain a crippling problem today, despite subsequent agreements to clean up the bay by 2000 and then by 2010.

William C. Baker, the bay foundation's president, said a judge could require the EPA to take drastic steps, such as setting a moratorium on all new permits to pollute in the watershed or cutting funding to states or cities that lag behind their cleanup plans.