ANNAPOLIS, FEB. 2 -- Environmentalists seeking to ban oil and gas drilling in the Chesapeake Bay and its tributaries received a boost from Maryland Gov. William Donald Schaefer and members of a key House committee today.

"We can't be inconsistent in trying to clean up the bay," Schaefer said in endorsing legislation banning drilling. Environmentalists and supporters of the drilling ban proposed by Del. Brian Frosh (D-Montgomery) were unsure which side Schaefer would take in the battle and said his support enhances the bill's chances.

Schaefer also said that he personally favored a permanent ban on the use of phosphate detergents, another priority of environmentalists. Maryland has been joined by the District and Virginia in banning phosphate detergents, but the state's prohibition is scheduled to end Dec. 1, 1989. A staunch pro-business politician, Schaefer opposed the ban when he was mayor of Baltimore, but said he has had a change of heart now that he is governor.

"Not changed," he said. "Now, I've become enlightened."

There is no drilling under way in the bay or its tributaries, but Texaco and Exxon recently began joint exploratory seismic testing for oil and gas in the Potomac River. The tests are scheduled to move to the bay, around Kent Island, next week.

Frosh said the drilling ban legislation is cosponsored by 15 of the 24 members of the House Environmental Matters Committee, which held a hearing on the proposal today. The committee will take a formal vote later in the 90-day session.

"We have asked many people to make sacrifices . . . in the hope the Chesapeake Bay will begin to heal itself," Frosh told the committee. "We cannot afford to let Exxon and Texaco play Russian roulette with this precious resource."

Seismic tests are the first steps in oil explorations, company officials said before the committee today. Exploratory, or "wildcat" wells would follow if the test results were encouraging, and setting up rigs would come next.

Under the proposed legislation, only the seismic tests would be allowed. The bay and river bottoms owned by Maryland could still be leased, under the proposal, but removing the oil or gas would be allowed only with underground drilling from operations on land, which oil company representatives said is not a common practice.

In the past, there has been little interest in exploring for oil in the bay or the surrounding land. But the oil companies began the tests in December, and while they won't disclose any preliminary findings, state officials said the companies are leasing lands in Southern Maryland and on the Eastern Shore.

The timing is bad, though. Disclosure of the testing plans came just as Schaefer, governors of surrounding states and federal officials signed a long-range agreement to clean up the bay. Schaefer said it would be inconsistent with the objectives of the pact to introduce the possibility of an oil spill.

Michael D. McDonald, executive director of the Maryland Petroleum Council, said that environmental groups have "exploited" that concern into seeking the ban.

McDonald said such groups were relying on "emotions rather than facts" and that a "total ban on drilling is not needed."

"The petroleum industry has an exemplary track record in operating in environmentally sensitive areas," McDonald told the committee. He added that "about 30,000 wells have been drilled in offshore state and federal waters without any recent major environmental incident."

The concern of the committee seemed to be that it would only take one accident, in the words of Frosh, "to set back all of our previous efforts {by} many years." Adding to the concern is that in estuaries such as the Chesapeake, water in the bay is not recharged quickly and an oil spill would linger, he said.

McDonald said it would be impossible for the companies to know whether oil was actually present without exploratory drilling.