Federal legislation to expand the Chesapeake Bay cleanup campaign over the next four years -- including new assaults on pollution from toxic chemicals and population growth -- was introduced yesterday by the region's congressional delegation.

The bill would pay for several new initiatives by increasing federal funding more than 50 percent, from the current $13 million a year to $20 million.

The Chesapeake Bay campaign is trying to reverse decades of pollution from farms, cities, suburbs and industry that has reduced the waterway's rich supply of fish, shellfish, birds and underwater vegetation.

There are promising signs -- fishing for striped bass, which had become increasingly scarce, was allowed this year after a five-year ban and Maryland's oyster catch looks better this season -- but most experts agree progress is fragile and real gains will not be seen for years.

Sen. Paul S. Sarbanes (D-Md.) cited recent oil spills, the spread of oyster disease, washups of medical waste and the persistence of toxins, nitrogen and phosphorus that choke aquatic life in the bay as evidence of continuing hard times for the nation's largest estuary.

"By various measures, there has been improvement," Sarbanes said at a news conference unveiling the bill, "but we're still far from where we want to be."

The bill would authorize the first report card to Congress on the bay program's progress, to be written by the Environmental Protection Agency and presented by 1994.

It proposes a new program to study the toxic chemicals such as metals and pesticides that flow to the bay from industry, sewage plants, lawns and farms, and runoff from farms and cities. Toxics are thought to be a major problem, but little research exists on their presence and impacts in the bay.

The legislation also would pay for a map of changing land use in the bay region, to help states curb the impact of an expected population boom. A related section would set up a trial program to identify and reduce major sources of urban and suburban runoff.

Forecasters predict the bay region's population will grow by 20 percent over the next few decades, bringing more pollution from construction, automobiles, home chemicals and other activities of daily life. Many bay experts say they fear those impacts will offset the impact of other efforts, such as sewage plant controls.

The legislation also includes

$1 million a year for a coordinated effort to study and undertake ways of bringing back the bay's supply of fish, shellfish and plants.

Environmentalists praised the bill, which was drafted with advice from the Chesapeake Bay Foundation. But some said they hoped for tougher controls on agriculture and military installations and other federal facilities.

"We would look at this as a good first draft," said Charles Fox, legislative director of Friends of the Earth.

The bay program bill is sponsored by Sarbanes and Rep. Roy P. Dyson (D-Md.), and co-sponsored by the entire Maryland and Virginia delegations, Del. Walter E. Fauntroy (D-D.C.) and several Pennsylvania and Delaware representatives. Introduced the day before Congress is scheduled to adjourn, it will be taken up next year as part of the proposed reauthorization of the federal Clean Water Act.