Cambridge politics will never be the same again.

For the first time in at least 20 years, three incumbent City Councillors are not seeking re-election. That by itself guarantees a shakeup, especially by comparison with 1987, when all nine councillors won re-election.

David E. Sullivan, the most outspoken proponent of rent control, announced his withdrawal from the 1989 council race this fall, citing a need to spend time with his family. And Saundra M. Graham, a fellow rent control supporter, says she is retiring from elected office to work with the League of Afro-American Women, a local minority entrepreneurship organization.

But the move that has rocked city politics most strongly is Mayor Alfred E. Vellucci's sudden decision to retire from the council and run for School Committee.

Swing Vote

Vellucci has spent most of his 34 years on the council providing the key swing vote for liberal programs such as rent control, while representing a conservative East Cambridge constituency.

For at least the past decade, the Council has been divided between four conservative Independents, with their commitment to neighborhood and business concerns, and four liberal Cambridge Civic Association (CCA) members, who favor rent control and limits on development.

Vellucci, however, has moved back and forth between the two camps, keeping the council as close to deadlocked as a nine-member body can be.

Members of all factions agree that Vellucci's absence means the November municipal election will produce a clear majority for one of the city's political coalitions for the first time since 1971.

While the six remaining incumbents appear secure, nobody wants to predict who will occupy the other three seats, or which political faction will gain the upper hand.

With Cambridge's at-large proportional representation system, the only sure things seem to be that another outspoken liberal will replace Sullivan and that an East Cambridge native--of uncertain political orientation--will replace Vellucci.

The smart money may be on the Independents, chiefly because no other candidate with Vellucci's liberal views seems capable of wooing his generally conservative East Cambridge constituents.

"I don't think there's any way the CCA can capture the hearts of the people of East Cambridge," says Independent Councillor William H. Walsh.

But Graham and Sullivan predict that the city's "progressives" might feel sufficiently threatened to get out the vote in unprecedented numbers and secure a fifth seat. Graham adds that the usual turnout for city council elections is around 35 percent of the electorate. In this election, she says creating a sense of urgency to get out the vote may be the most important task for either side.

If the CCA wins five seats--as it last did in 1971--it will most likely use its clear majority to enact stricter curbs on developers. It could also refuse to renew the contract of City Manager Robert W. Healy, an Independent ally, and it might investigate Independent-dominated city departments such as Inspectional Services.