FOR THE PAST several years rent control has protected Cambridge residents from the skyrocketing rents that an accelerating influx of students and young professionals would otherwise have created. But now the conversion of housing units into condominiums provides landlords with a means of by-passing rent control, allowing them to reap hefty profits while they evict occupants and reduce the amount of housing available to low- and middle-income tenants.

Last week the Cambridge City Council adopted the weakest of six proposals to slow the conversion trend. The decision represented an ineffectual move against one of the city's most pressing problems. By requesting the state legislature to order an immediate six-month stay on evictions of any elderly people whose apartments are slated for conversion, the council has skirted the issue and tried to offer an election-year appeasement to elderly voters, through a move conversion opponents consider an insult. Landlords and those who purchased condominiums because they cannot afford houses deserve some consideration, but strong action must nonetheless be taken to prevent an economically forced exodus of working people from Cambridge.

Last week's council vote showed a usual split: the three councilors running as part of liberal Cambridge Convention '77 and Mayor Alfred E. Vellucci vainly supported curbs on conversion, standing against the independent majority. David Clem, a renegade member of Cwmbridge Convention '75, proved to be the council's most disappointing member when he failed to support stronger curbs.

Although the vote is a definite set-back for those who want to protect Cambridge's low- and middle-income residents, it has a brighter side. It further clarifies the choice voters will have in next month's city election.