The House of Delegates gave preliminary approval today to legislation that would ban the sale and distribution of phosphate detergents in Maryland as proponents repelled several attempts to weaken the already diluted bill.

During a debate that lasted more than two hours, opponents of the phosphate ban unsuccessfully blasted a bill they argued had been so amended in committee as to be nonsensical. As currently written, the legislation allows homeowners with septic tanks, customers of coin-operated laundries and veterinary hospitals among others to use detergents containing phosphates but prohibits them from purchasing the products in Maryland.

"We are faced with the absurd public policy of telling 35 percent of the people in this state they can use these products but that to use them they have to bootleg them in from out of state," said Del. Casper R. Taylor Jr. (D-Western Maryland).

But supporters of the ban, who argue that the bill would aid the effort to restore water quality in the Chesapeake Bay, managed to defeat several crippling amendments, including proposals to permit the sale of such detergents to residents who are exempted, and one attempt to delay implementation of the ban until neighboring states enact similar legislation.

House members, who have been subjected to a relentless lobbying campaign by detergent, chemical and textile interests, appear narrowly divided on the legislation heading into a final vote, which is scheduled for Thursday. A head count by supporters last night showed 67 votes for the measure, four shy of what is needed to pass the full House.

However, proponents of the phosphate ban were buoyed by the support they found today and appeared to benefit from a lobbying effort that some House members say has been heavy-handed and counterproductive.

"It's going to be awfully close, but I remain optimistic," said Del. William R. McCaffrey (D-Prince George's), who managed the bill on the floor.

McCaffrey compared the lobbying blitz by the dozen or so industry representatives who are trying to defeat the bill to "a Chinese fire drill. The one thing it has not been is coordinated."

Supporters of the bill, a more stringent version of which has already passed the Senate, had to settle for a heavily amended bill in committee as the price of getting the measure to the floor. For example, the exemption for veterinarians was a concession to Del. James E. McClellan (D-Frederick), a veterinarian who sought the same exemption given to hospitals in the Senate bill. Also added in committee was a three-year limit on the phosphate prohibition unless the legislature extends it.

But opponents of the ban used the numerous exemptions in the bill to score rhetorical points during today's debate, ridiculing apparent incongruities in the legislation.

Addressing the exemption for coin-operated laundries, without which that industry said it would face increased costs for maintaining machinery, Del. Jerry H. Hyatt (D-Montgomery) said: "I didn't know we were trying to save washing machines. I thought we were trying to save the bay."