The line has begun to form: Rep. Robert T. Matsui (D-Calif.) announced his candidacy for the Senate seat held by Alan Cranston (D-Calif.), calling for a "return to idealism." Cranston, who is fighting prostate cancer and is one of the senators being investigated by the Senate ethics committee in the "Keating Five" case, recently announced that he will not seek reelection in 1992.

Matsui, a 12-year House member, did not mention Cranston's involvement in the ethics probe, but he called for "a higher level of ethics in public office." He said "too many Americans . . . believe government is no longer about public service, but self-interest."

Matsui, who is not well-known outside Sacramento and the Central Valley district that he represents, hopes his early announcement will boost him statewide.

A crowd of House members of both parties along with other state officials and former officials is expected to vie for the two Senate seats that will be open in California in 1992. The second Senate election will be to complete the term of Sen. Pete Wilson (R), who was just elected governor.

Matsui, a highly successful fund-raiser thanks in part to his seat on the House Ways and Means Committee and his support from Japanese and other Asian Americans, has $1.3 million in the bank as seed money for a Senate race that will cost at least $10 million.