SAN MARCOS, Calif.—He could defend himself from a federal investigation over whether he used his political position to wrongly enrich himself and he could run successfully for office again, but Rep. Randy “Duke” Cunningham concluded he couldn’t do both at the same time.

That, Cunningham decided Thursday, was the reason he won’t be running for election to a ninth term in Congress next year.

“The time has come for me to conclude the public chapter in my life,” said Cunningham, his wife, Nancy, by his side. “Quite simply, right now I may not be the strongest candidate.”

Cunningham, whose term ends in January 2007, read a brief written statement and did not take questions before hugging supporters and ducking back into a library on the California State University, San Marcos, campus, north of San Diego.

“What Duke considered he had in front of him was the challenge of meeting this investigation, serving his constituents, and at the same time mounting a re-election campaign. And he figured all three of those were going to be very difficult to carry out effectively,” said House Armed Services Committee Chairman Duncan Hunter, R-Alpine, who represents an adjoining district and is among Cunningham’s closest friends in the House.

The former Navy “Top Gun” fighter pilot and Vietnam War veteran has been hounded the past month by questions concerning his relationship with longtime friend Mitchell J. Wade, the founder of defense firm MZM Inc.

Federal prosecutors have been investigating Cunningham’s 2003 sale of his 3,826-square-foot house in the wealthy coastal community of Del Mar to Wade for $1.675 million.

Wade resold the house the following year at a $700,000 loss. That amounted to a 60 percent drop in value during a period when the average price of a San Diego County home increased 25 percent.

Around the same time, Washington, D.C.-based MZM was increasing its federal contracting business. Revenues tripled last year, according to the company’s Web site. In fiscal 2004, MZM was No. 38 on the list of defense contractors performing intelligence-related and other work, with $65.5 million of contracts.

Cunningham, 63, is a member of the House Appropriations defense subcommittee and the Permanent Select Committee on Intelligence, both of which oversee the kind of classified intelligence work MZM does.

As he has in the past, Cunningham conceded Thursday that he showed “poor judgment” in selling his home to Wade but insisted any investigation would exonerate him. He said he did not profit improperly from the sale.

After selling the home, Cunningham bought a $2.55 million seven-bath mansion in Rancho Santa Fe, which census data rank as the nation’s wealthiest community of at least 1,000 households.

He said Thursday he plans to sell the Rancho Santa Fe home and donate a portion of the proceeds to three San Diego charities: a homeless shelter, a school for at-risk children and a home for battered women.

The congressman also faces questions about an arrangement with Wade under which he lived on the businessman’s boat when it was moored at a Washington, D.C., yacht club. The boat left the Capital Yacht Club on Wednesday and will be sold.

Cunningham has said that instead of rent he has paid at least $13,000 to cover dock fees and other expenses since April 2004. Living on Wade’s yacht for free would violate congressional ethics rules.

Late last month the FBI searched the boat, which is named the Duke Stir, as well as Cunningham’s Rancho Santa Fe home and MZM’s Washington, D.C. headquarters. A federal grand jury in San Diego has subpoenaed documents from Cunningham.

The congressman, who is staunchly conservative, has fit his district well, winning his 2004 re-election with 58 percent of the vote.

He said Thursday his instinct had been to run again so he could fight for causes he believes in, such as national defense, medical research, schools and local job creation. Despite his reputation as a tough military man, Cunningham has been known to tear up while speaking on the House floor about issues important to him.

“Today he did for his party what he’s always done for his country _ put the interest of others above his own,” said Carl Forti, spokesman for the National Republican Congressional Committee, the GOP’s congressional fund-raising arm. Forti said Cunningham was not pressured by party leaders to retire.

Cunningham’s district is strongly Republican and GOP leaders are confident they will retain the seat. Democrats believe they have a chance to take it.

“What led to Congressman Cunningham’s announcement today is exactly why the American people hold Congress in such low esteem,” said Bill Burton, spokesman for the Democratic Congressional Campaign Committee. “The American people deserve a Congress that looks out for their pocketbook issues, not their own pocketbooks.”

Cunningham’s 2004 Democratic opponent, Francine Busby, plans to run again. On Thursday, she said he should resign immediately. So did the Washington-based political watchdog group Citizens for Responsibility and Ethics.

Cunningham conceded that his announcement would not sway his critics.

“This gesture will not convince the skeptics of our good faith,” he said. “My detractors have made up their minds, and this act will not reach them—or in any way end the government’s investigation of me.”

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Associated Press Writer Erica Werner in Washington, D.C., contributed to this report.