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, visiting his offices and accompanying him on 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 client, 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.

Stephan Savoia John McCain and his political advisor John Weaver (left).

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.)

Mr. McCain helped found a nonprofit group to promote his personal battle for tighter campaign finance rules. But he later resigned as its chairman after news reports disclosed that the group was tapping the same kinds of unlimited corporate contributions he opposed, including those from companies seeking his favor. He has criticized the cozy ties between lawmakers and lobbyists, but is relying on corporate lobbyists to donate their time running his presidential race and recently hired a lobbyist to run his Senate office.

"He is essentially an honorable person," said William P. Cheshire, a friend of Mr. McCain who as editorial page editor of The Arizona Republic defended him during the Keating Five scandal. "But he can be imprudent."

Mr. Cheshire added, "That imprudence or recklessness may be part of why he was not more astute about the risks he was running with this shady operator," Charles Keating, whose ties to Mr. McCain and four other lawmakers tainted their reputations in the savings and loan debacle.

During his current campaign for the Republican presidential nomination, Mr. McCain has played down his attacks on the corrupting power of money in politics, aware that the stricter regulations he championed are unpopular in his party. When the Senate overhauled lobbying and ethics rules last year, Mr. McCain stayed in the background.

With his nomination this year all but certain, though, he is reminding voters again of his record of reform. His campaign has already begun comparing his credentials with those of Senator Barack Obama, a Democratic contender who has made lobbying and ethics rules a centerpiece of his own pitch to voters.

"I would very much like to think that I have never been a man whose favor can be bought," Mr. McCain wrote about his Keating experience in his 2002 memoir, "Worth the Fighting For." "From my earliest youth, I would have considered such a reputation to be the most shameful ignominy imaginable. Yet that is exactly how millions of Americans viewed me for a time, a time that I will forever consider one of the worst experiences of my life."