ANY MARYLANDER worried about the condition of the Chesapeake Bay should keep a close eye on how each member of the house of delegates votes in Annapolis this week on a bill that would ban phosphates from detergents in the state. Those lawmakers who are serious about saving the bay should recognize and resist the tactics of the detergent lobby -- namely, to weaken the house version of the bill enough to provide an excuse to kill it. No soap. Delegates should approve a bill, even if it is weak, to allow for a compromise with the Senate. To vote against a bill in the house would be to kill any such proposal for the year.

The detergent lobby would have you believe that phosphorus entering the bay (a) is really no big pollution problem, or (b) can wait until the state's sewage treatment facilities can remove most phosphorus from everything, including detergents. The Maryland senate knew enough to reject both of these arguments. the votre was 39 to 5 for a ban.

What these state legislators knew was that phosphorus is a fertilizer. In the fresh-water protions of the Chesapeake Bay, phosphorus contributes to excess growth of algea. These algae die and decrease the dissolved oxygen in the water. That kills off submerged vegetation, shellfish, fin fish and other things that are considered critical to the water's quality. The lawmakers who voted for the ban also knew that there are phosphate-free detergents that work just fine.

The Soap and Detergent Association arues that the bill would deny household consumers what it calls ''freedom of choice in purchaisng cleaning products.'' But more than a few household consumers are keenly aware of a far more important choice for the future: either take every reasonable step to curb pollution of the bay and other waters, or find excuses not to. Despite the failure so far of house leaders to rally votes for the phosphate ban, individual delegates should see the importance of supporting a bill that would serve as a start on even more effective moves to help save the bay.