In their first and only scheduled debate, two Republican candidates for U.S. Senate, John Spencer and Kathleen Troia “KT” McFarland, tore into each other with a viciousness that has typified their long-shot campaigns to unseat Senator Clinton.

At Pace University, Mr. Spencer, a blunt-spoken former mayor of Yonkers, faced withering attacks by Ms. McFarland, a self-described Cold Warrior turned Upper East Side matron, for his affair with a top aide while in office. Mr. Spencer accused her of spreading “scurrilous” lies and innuendoes and rebuked her for taking a scalpel to his personal life.

For the candidates, who are struggling to generate campaign contributions and public attention, the debate fit into the Republican primary campaign’s war of attrition, which has seemed like a bizarre sideshow to Mrs. Clinton’s campaign — one many view as a prelude to a presidential bid.

Compared to the attacks on each other, the candidates’ criticism of Mrs. Clinton seemed tame. Ms. McFarland said Mrs. Clinton failed to clearly define her position on the Iraq war, while Mr. Spencer said the senator’s liberal politics were a destructive force in America. Many of his attacks were aimed at the “liberal press” and the Journal News, which was critical of him during his two terms as mayor.

The most scornful line of the night was delivered by Ms. McFarland with a pre-packaged articulacy. As a retort to accusations that her campaign is aiding Mrs. Clinton, she said, “You keep referring to me as being like Hillary Clinton. You have since the first day I got in the campaign. John, the problem with you is that you are like the Clintons. You taxed and spent like Hillary and you behaved like Bill.”

She cast her attack not as a personal strike against Mr. Spencer, who fathered two children with the aide before he was divorced from his wife, but as an indictment of his public record.

“Had you been in the military and behaved that way, you would have been court-martialed. If you worked in the federal government and behaved that way, you have been subject to indictment,” Ms. McFarland said. “Those issues are perfectly reasonable to bring up because they speak to your ethical standards, conduct, and fiscal responsibility.”

The candidates appeared to be most careful when talking about the White House. Asked to grade President Bush’s war-time performance, Ms. McFarland, a Pentagon official during the Reagan administration, said: “There have been mistakes made in the Iraq war. In retrospect, we didn’t send in enough troops.” She added: “It’s not my place to tell a president what to do with regard to Secretary Rumsfeld.” Mr. Spencer also was reluctant to attack the president: “You don’t pitch no-hitters in war,” he said.

Both candidates interrupted each other with abandon and provided answers that spiraled into rambling anecdotes that were often cut off by the panel of questioners. Ms. McFarland spoke with a cool, quiet ease that belied the harshness of her language. He often reacted to Ms. McFarland with extended guffaws.

Her decision to assail Mr. Spencer for his marital infidelity was born of a premeditated strategy. Her goal seemed to be to keep the campaign personal but turn the unflattering attention to her opponent and to dare Mr. Spencer to fight back by bringing up her troubled past involving recovered memories of family abuse that have become tabloid fodder.

Mr. Spencer declined, and instead declared himself shocked by her invasion of his personal life. “She has insulted my wife. She has insulted my children,” he said.