The Michigan attorney general, a former Marine, was teary-eyed. He said his actions were "inexcusable."

"A number of years ago, I was unfaithful to my wife, Laura," Mike Cox told reporters in Detroit. "I am completely responsible for what happened -- it was entirely my fault."

It was not, however, Cox's initiative that drove the facts into the public domain.

The rising Republican star said last week that a political rival had tried to blackmail him. The opponent was Geoffrey Fieger, a Democratic lawyer and aspiring politician best known for representing assisted-suicide advocate Jack Kevorkian.

"I will not let a bully," Cox declared with his wife, a fellow politician, by his side, "prevent me from doing the job the people of Michigan elected me to do."

Fieger, who lost spectacularly in the 1998 governor's race, has a number of beefs with Cox.

For one thing, Cox has been investigating Fieger's role in a $457,000 advertising campaign last year designed to defeat a Michigan Supreme Court justice. For another, Fieger said he would run against Cox in 2006. He also sued him.

"I have been blamed for a lot of things," Fieger said after Cox's news conference, "but this is the first time I have been blamed for another man cheating on his wife."

Fieger denies doing anything improper. A decision may rest with voters and the Michigan Attorney Discipline Board, which oversees the state's lawyers. Oakland County Prosecutor David Gorcyca announced Monday that he would send his findings to the board after concluding the evidence was too thin to sustain criminal charges.

Gorcyca did say the evidence "soundly convinces me that a severe and reprehensible ethical violation or violations were committed." He added that Fieger and an alleged accomplice named Lee O'Brien should not "claim victory, act virtuous or gloat. Far from it."

This is a case difficult to follow without a scorecard, but it is capturing plenty of attention among Michigan's political classes.

Cox produced a taped telephone message that he said came from O'Brien, who is also a lawyer.

In the recording, O'Brien tells a Cox staff member, "Fieger wants me to deliver a threat to your boss. You or him give me a call; you know what I mean."

O'Brien allegedly said later that Fieger would reveal Cox's extramarital affair unless the attorney general dropped the investigation into the television ad campaign. The prosecutor's office, according to Cox, monitored two meetings before deciding the evidence was not strong enough.

It remains unclear what the tangle means for the original investigation into whether Fieger broke campaign finance laws by backing an expensive campaign against Supreme Court Justice Stephen J. Markman, who had, in a further complication, been supported by Cox.

Cox "ought to recuse himself," said Bill Ballenger, editor of Inside Michigan Politics. "Cox's ability to conduct a legitimate investigation into the charge against Fieger seems hopelessly compromised."

It is also unclear what this means for the political future of Cox, 44, who has been assumed to be standing in line for a shot at the governor's mansion. He intends to run for reelection as attorney general next year.

Cox won support in 2002 when he spoke of raising a daughter, born out of wedlock, after his former girlfriend decamped when the girl was 3.

In his mea culpa last week, Cox said he told his wife, a Wayne County commissioner, of the affair in March 2003.

"Through a great deal of work," Cox said, "we have made our marriage stronger than ever."

Mike Cox, Michigan's attorney general.