When The Boston Globe disclosed the Paxson intervention a few weeks after the Claremont summit, McCain handled the disclosure with aplomb, even chutzpah. The chairman of the Senate Commerce Committee argued that he had done nothing wrong and that the suspicion falling on him only reinforced his argument for campaign finance reform.

"We're all tainted," McCain said. "We're all under suspicion as long as Washington is awash in special interest money."

Early in Senator John McCain’s first run for the White House eight years ago, waves of anxiety swept through his small circle of advisers.

A female lobbyist had been turning up with him at fund-raisers, in his offices and aboard a client’s corporate jet. Convinced the relationship had become romantic, some of his top advisers intervened to protect the candidate from himself — instructing staff members to block the woman’s access, privately warning her away and repeatedly confronting him, several people involved in the campaign said on the condition of anonymity.

When news organizations reported that Mr. McCain had written letters to government regulators on behalf of the lobbyist’s clients, the former campaign associates said, some aides feared for a time that attention would fall on her involvement.

Mr. McCain, 71, and the lobbyist, Vicki Iseman, 40, both say they never had a romantic relationship. But to his advisers, even the appearance of a close bond with a lobbyist whose clients often had business before the Senate committee Mr. McCain led threatened the story of redemption and rectitude that defined his political identity.

It had been just a decade since an official favor for a friend with regulatory problems had nearly ended Mr. McCain’s political career by ensnaring him in the Keating Five scandal. In the years that followed, he reinvented himself as the scourge of special interests, a crusader for stricter ethics and campaign finance rules, a man of honor chastened by a brush with shame.

But the concerns about Mr. McCain’s relationship with Ms. Iseman underscored an enduring paradox of his post-Keating career. Even as he has vowed to hold himself to the highest ethical standards, his confidence in his own integrity has sometimes seemed to blind him to potentially embarrassing conflicts of interest.

Mr. McCain promised, for example, never to fly directly from Washington to Phoenix, his hometown, to avoid the impression of self-interest because he sponsored a law that opened the route nearly a decade ago. But like other lawmakers, he often flew on the corporate jets of business executives seeking his support, including the media moguls Rupert Murdoch, Michael R. Bloomberg and Lowell W. Paxson, Ms. Iseman’s client. (Last year he voted to end the practice.)