Rep. Bobbi Fiedler (R-Calif.), a leading candidate for the Republican Senate nomination, and her closest aide have been indicted for allegedly offering a GOP primary opponent $100,000 to quit the race, the Los Angeles County grand jury announced today.

The indictment further confused a tight contest for the right to challenge Sen. Alan Cranston (D-Calif.) next fall. State Sen. Ed Davis, who said intermediaries offered him the money to pay off campaign debts, has held a narrow lead in the polls over at least nine other announced or potential candidates. Fiedler, like Davis, represents the affluent and conservative San Fernando Valley.

"This whole thing is ridiculous," a weary-looking Fiedler said at a packed news conference today. "I've done nothing wrong, and I don't believe anyone in my campaign has done anything wrong."

The indictment, issued Thursday and unsealed today, also names Fiedler's administrative assistant and political adviser, Paul Clarke. "What you are witnessing is one of the greatest political dirty tricks of all time," said Clarke, who declined to identify the culprit.

[The indictment was based in part on a conversation secretly recorded by Davis' campaign manager, Martha Zilm, as she and Clarke visited Fiedler's home Jan. 12, the Los Angeles Times reported. Sources who told the Times about the recording said they did not know details of the conversation, the newspaper said.]

A source close to the Fiedler campaign said it was difficult to comment on the charges because staffers had not seen the grand jury transcript. The source said Fiedler staffers felt that they had been careful to stay within the law in their overtures to Davis but declined to elaborate.

The investigation of Fiedler's campaign was conducted by the office of Los Angeles District Attorney Ira Reiner, a Democrat who was city attorney before his election in 1984. Deputy District Attorney Steven Sowders, who led the investigation, said the political implications of charging Fiedler were never considered.

Fiedler, a three-term representative, appeared briefly in court this morning to hear the indictment, but her attorney asked that arraignment be delayed until Monday. She and Clarke said they expected to be cleared but would follow their attorneys' advice to refrain for now from commenting on the specific charges.

Fiedler and Clarke first gained political prominence here in the 1970s through their successful efforts to stop massive busing of white students to increase integration of the Los Angeles public schools.

They are charged under a California law that makes it a felony to "directly or through any other person advance, pay, solicit or receive or cause to be advanced, paid, solicited, or received any money or any other valuable consideration . . . . to induce a person not to become or withdraw as a candidate for public office."

Sowders said a conviction could bring a maximum three-year prison sentence. He declined to comment on a report from the Fiedler camp that the grand jury had voted to indict against the district attorney's recommendation.

Davis said today that one of his longtime supporters acted as a reluctant emissary for the Fiedler campaign and contacted a Davis aide in early November to ask whether Davis might drop out if Fiedler promised to pay off his campaign debt, then $100,000.

Davis, a former Los Angeles police chief, said he was surprised that someone outside his campaign knew the amount of his debt. Davis said he considered the offer "nonsense" because he was leading the race.

Suspecting a violation of the law, Davis said, he consulted private attorneys and then Ventura County District Attorney Michael Bradbury, one of his supporters. Bradbury said he alerted his own investigators, who determined that Los Angeles prosecutors had jurisdiction. Sowders said his team interviewed "five or six witnesses," whom he would not identify, before going to the grand jury.

According to a Mervin Field California poll conducted in late November, Davis, 69, had a narrow lead over three other well-known conservatives competing in the June primary. Davis had 12 percent, Los Angeles County supervisor Mike Antonovich 11 percent, television commentator Bruce Herschensohn (who announced his candidacy on the air Thursday night) 10 percent and Fiedler 9 percent.

Fiedler, 48, appeared to have had more fund-raising success than any of the front-runners. Her aides say she had collected more than $700,000, compared to about $550,000 for Davis, who has had to explain to fellow conservatives his support of a state law prohibiting discrimination against homosexuals.

Fiedler said she planned to continue her campaign. David Narsavage, a spokesman for the National Republican Senatorial Committee, told the Associated Press in Washington that the indictment probably ruins Fiedler's chances.

Others mentioned in the California poll were actor Fess Parker, 4 percent; state assemblyman Bob Naylor, 4 percent; supply-side economist Arthur Laffer, 4 percent; Rep. Edwin V.W. Zschau, 3 percent; Rep. William E. Dannemeyer, 3 percent, and Rep. Daniel E. Lungren, 2 percent.

Despite Cranston's energetic constituent service staff and big majorities in previous elections, Republicans consider him vulnerable because of his age, 71, and his loyalty to liberal Democratic policies in a state where conservatives are well-organized. Even without today's charges, which some campaign officials said would dry up Fiedler's funds, many experts wondered whether she could survive a primary while supporting the Equal Rights Amendment and opposing a constitutional amendment banning abortion.

With so many southern Californians in the race, some consultants expected Zschau, a Silicon Valley businessman, to join the front-runners with his fund-raising success, his northern California base and an early television campaign.