Sen. Alan Cranston (Calif.), the Senate's second-ranking Democratic leader and a key figure in the "Keating Five" ethics investigation, announced yesterday that he has prostate cancer and will not seek reelection as majority whip next week or election to a fifth term in the Senate in 1992.

In a statement from his Washington office, the 76-year-old Californian said his physicians regard the cancer, which was confirmed Oct. 15, as "apparently totally curable" but say it requires immediate treatment, possibly including surgery or radiation therapy.

Cranston's announcement comes in the wake of reports that, largely because of the ethics probe, he was in danger of losing both the leadership post that he has held since 1977 and the Senate seat he has occupied since 1969.

Sen. Wendell Ford (D-Ky.), who ran unsuccessfully against Cranston for the whip job two years ago and was challenging him again this year, was substantially ahead of Cranston in nose-counts for the Senate leadership elections Tuesday, according to several senators.

Ford has reportedly told colleagues he has at least 35 votes; Cranston said in his statement he had commitments from 19 senators.

Exit polls of Californians after they voted this week indicated that only 10 percent thought Cranston should run again, with 33 percent saying he should not run again and 24 percent urging that he resign now. Mervin Field's California Poll reported in September that 74 percent were not inclined to vote for him, with 32 percent saying he should resign immediately.

Cranston's departure from the whip's race could draw Sen. David Pryor (Ark.), now secretary of the Democratic Conference, the third-ranking Democratic leader, into the whip's race, according to a colleague who described Pryor as "exploring the possibility of running."

Pryor, who was out of town and unavailable for comment, is understood to be leaning toward entering the race if he sees a reasonable chance of winning.

Pryor is especially popular with some of the Senate's newer members and, as a southerner, would give the leadership the same regional balance as Ford. He had ruled out a bid for the whip post as long as Cranston was a contender.

As a member of the Senate ethics committee, Pryor will be called on to rule on allegations that Cranston and four other senators intervened improperly on behalf of savings and loan executive Charles W. Keating Jr. while receiving contributions from Keating for their campaigns and favorite causes. The other four are Sens. Dennis DeConcini (D-Ariz.), John Glenn (D-Ohio), John S. McCain III (R-Ariz.) and Donald W. Riegle Jr. (D-Mich.).

Trial-like public hearings in the case are to begin Thursday and are expected to last several weeks. The committee has said it hopes to make a decision in the case by year's end. It could drop proceedings against any or all of the senators, issue reprimands or recommend that the Senate as a whole take more serious steps, which could include censure or even expulsion.

Cranston's illness is not expected to delay the hearings. A Cranston aide said the senator plans to attend the hearings Thursday and Friday and is not requesting a postponement.

Cranston's decision to retire in 1992 enlivens what already was expected to be a wide-open battle for two Senate seats in California that year, including the seat now held by Pete Wilson (R). As the newly elected governor, Wilson can choose his own successor, whose appointment must be confirmed by voters in 1992.

For California politicians entrapped by new term limits passed by voters last Tuesday, the double shot at Senate seats could not have come at a better time.

Former San Francisco mayor Dianne Feinstein (D), who ran a strong but losing campaign for governor, is seen as an almost certain contender. Other Democratic possibilities include former Gov. Edmund G. "Jerry" Brown Jr., now state party chairman; Reps. Mel Levine, Robert Matsui, Barbara Boxer and Nancy Pelosi; state controller Gray Davis and lieutenant governor Leo McCarthy.

Republican possibilities for appointment to Wilson's seat or candidacy for Cranston's seat include U.S. trade representative Carla A. Hills, Energy Secretary James D. Watkins, former representative Edwin Zschau and state Sens. John Seymour and Rebecca Q. Morgan, along with Reps. David Dreier, Robert K. Dornan, William Thomas and Jerry Lewis. Also mentioned is news commentator Bruce Herschensohn.

Cranston, who was an unsuccessful contender for the Democratic presidential nomination in 1984, said his illness was first detected by Senate physicians in a routine annual examination and then confirmed in a biopsy at Walter Reed Hospital. He said he has consulted cancer experts in New York and plans further consultations in the next few days.

Staff writer Jay Mathews in Los Angeles contributed to this report.