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.”

A drive to expunge the stain on his reputation in time turned into a zeal to cleanse Washington as well. The episode taught him that “questions of honor are raised as much by appearances as by reality in politics,” he wrote, “and because they incite public distrust they need to be addressed no less directly than we would address evidence of expressly illegal corruption.”

Image The lobbyist Vicki Iseman, whose relationship with Mr. McCain troubled some of his aides. Credit... Stephen Boitano/Getty Images

A Formative Scandal

Mr. McCain started his career like many other aspiring politicians, eagerly courting the wealthy and powerful. A Vietnam war hero and Senate liaison for the Navy, he arrived in Arizona in 1980 after his second marriage, to Cindy Hensley, the heiress to a beer fortune there. He quickly started looking for a Congressional district where he could run.