WASHINGTON, July 14 - Under investigation for a real estate deal and his ties to a military contractor, Representative Randy Cunningham, an eight-term Republican, announced Thursday that he would not seek re-election next year in his district in the San Diego area.
"I do not believe that a political campaign in the midst of such an investigation is in the best interests of my family or my constituents," Mr. Cunningham said at a hastily called news conference at which he denied any wrongdoing but acknowledged poor judgment.
The decision by Mr. Cunningham, a conservative and former Navy pilot active on military and intelligence issues, makes him the first lawmaker to be claimed by a swirl of ethics accusations that have engulfed the House this year.
Representative Tom DeLay, the majority leader, has come under intense scrutiny over his travel as well as his ties to the lobbyist Jack Abramoff, who is the subject of Congressional and federal inquiries. Other lawmakers of both parties have raced to file or correct their own travel records in light of the focus on Mr. DeLay, a Texas Republican.
Democrats have also questioned the activities of a handful of other Republicans and cited the investigation that led to Mr. Cunningham's decision not to run as a reason that public regard for Congress is slipping.
"The American people deserve a Congress that looks out for their pocketbook issues, not their own pocketbooks," said Bill Burton, the communications director for the Democratic Congressional Campaign Committee.
Republicans dismissed any suggestion that Democrats would be able to capitalize in next year's elections on the House ethical climate.
"I don't know of any member of Congress who has ever lost a race because of something another member of Congress allegedly did or didn't do," said Carl Forti, a spokesman for the National Republican Congressional Committee. If Democrats seek to focus on ethics, Mr. Forti said, Republicans will be ready to air accusations about Democratic leaders.
Mr. Cunningham's problems began earlier this summer when the Copley News Service disclosed that a defense contractor who is close to him bought Mr. Cunningham's home for $1,675,000 in 2003 and sold it nearly a year later at a loss of $700,000 in one of the nation's hottest housing markets.
Mr. Cunningham was also living in Washington on a yacht owned by the contractor, Mitchell J. Wade, a founder of the firm MZM, Inc. The firm has been getting more federal military-related business in recent years, and Mr. Cunningham is on a subcommittee that oversees military spending. Federal agents recently searched Mr. Cunningham's home, the yacht and the firm's Washington office.
On Thursday, appearing with his wife, Nancy, on the campus of California State University, San Marcos, Mr. Cunningham read a prepared statement, his voice breaking occasionally.
"I fully recognize that I showed poor judgment when I sold my home in Del Mar to a friend who did business with the government," he said. "I should have given more thought to how such transactions might look to those who don't know me like you do."
Mr. Cunningham said that he expected to be vindicated but that he had decided it was best to step aside. He also said he and his wife intended to sell their new house and donate some of the proceeds to local charities.
"I learned in Vietnam that no one person is more important than the mission, and I do not intend to forget that lesson now," he said, adding that it would be best to allow someone else to represent the district, the 50th. He took no questions.
The Republicans expect to be able to retain the seat, which is in an area that gave President Bush 55 percent of the vote in the last election.
"Today, Duke Cunningham did for his party what he has always done for his country," said Representative Thomas M. Reynolds of New York, chairman of the House Republican campaign committee, using Mr. Cunningham's nickname. "He put the interests of others above his own ambitions."