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.

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"I would very much like to think that I have never been a man whose favor can be bought,"