A Teacher for Senator: Presidential candidate Sen. Alan Cranston (D-Calif.) has won the endorsement of the president of the Iowa affiliate of the National Education Association, which could complicate former vice president Walter F. Mondale's hope of having the teacher's union behind him in the state that has the first presidential caucuses of 1984.

Phoebe Tupper, breaking with other teachers' union officials who had agreed to remain neutral in the presidential race until the NEA makes its endorsement next week, decided to support Cranston personally.

Tupper, who traveled with Mondale during his announcement swing through Iowa last February, has been dispatched to Maine by the Cranston campaign to lobby teachers there before the straw poll to be held Saturday.

Mondale is expected to get the NEA endorsement Thursday. But, the affiliates of each state must ratify the endorsement decision before the national group can work in states in support of the candidate it endorses.

The Iowa State Education Association political action committee will vote on whether to concur with the NEA endorsement on Nov. 19.

A Mondale campaign official said yesterday that Tupper's endorsement of Cranston was expected, but expressed confidence that they would still receive the support of the teacher's union in Iowa.

He said that Tupper's early endorsement of Cranston had angered other members of the ISEA PAC board.