The Cambridge City Council last night began consideration of a proposal to gradually phase out Cambridge's rent control program.

Councilor Walter Sullivan's call for vacancy decontrol--tabled for a week--is the first legislative challenge to rent control in more than a year. His proposal would end the limits on the rent a landlord can charge for an apartment as soon as current tenants leave their homes, effectively ending rent control gradually as the city's apartments change hands.

A slim 5-4 majority on the council has maintained the city's rent control program for several years, and Sullivan said last night he had "no indication" that any votes on the board had changed.

"I'm just sick and tired of us taking care of everyone who is moving into the city and taking advantage of rent control," Sullivan said. Current tenants need rent control, he added, "but we shouldn't be subsidizing outsiders."

Rent control proponents said yesterday they thought support for the controversial plan remained firm, but predicted a flurry of new attacks from city landlords and developers.

"It's a good time for the opponents of rent control to make their move--they are using Reagan's threats to cut off federal aid to cities with rent control, and the need to cut costs after Proposition 2 1/2, as excuses to sacrifice rent control," City Councilor David Sullivan said yesterday.

Sullivan said that a vacancy decontrol program would encourage landlords to harass tenants in an effort to drive them out so prices could be raised. In addition, vacancy decontrol would mean a "slow death" for rent control in the city, he added.

Cambridge adopted rent control in the early 1970s in an effort to preserve low and moderate income rental housing. Critics have since attacked the law for maintaining rents at below-market prices, and called the city's rent control administration slow and inefficient.

The four councilors endorsed by the liberal Cambridge Civic Association (CCA) are pledged to support Cambridge's rent control program. The fifth vote on the nine-member council in favor of rent control has been cast over the years by Alfred E. Vellucci, an independent councilor from East Cambridge.