Fourteen liberal candidates for the Cambridge City Council and School Board will seek endorsement for this fall's elections Saturday at Cambridge Convention '77, a rally in Central Square that convention officials said yesterday is meant to unite Cambridge liberals behind a slate of candidates.

A coalition of several Cambridge liberal political groups is organizing the convention at which all registered Cambridge voters may participate, Robert W. Smith '69, a member of the convention's steering committee, said yesterday.

Candidates for endorsement must sign a pledge supporting 14 aims for change in Cambridge government, including strict rent control and more jobs for women and minority group members, Smith said.

Smith said that because Cambridge's system of proportional representation makes it possible for someone to be elected with a minority of the vote, by allowing votes to list candidates on the ballot in order of preference, the convention committee hopes that five of its nominees will be elected in November.

David E. Sullivan, a third year law student seeking convention endorsement in his campaign for a position on the city council, said yesterday he thinks the convention will endorse him because of his past political work in Cambridge and because he has organized a campaign staff that includes several Harvard students to support him.

Sullivan, the only Harvard student running for local office, is a member of the state Democratic committee and was Massachusetts student coordinator for Sen. George S. McGovern's 1972 presidential campaign.

Dana M. Stein '80, a member of the convention committee, said yesterday he is mailing notices of Saturday's rally to approximately 600 Harvard students who worked in last fall's Carter-Mondale campaign or have registered and vote in Cambridge.