In late 1989, Senator John McCain went home to Arizona to fight for his political life.

It was at the height of the savings and loan scandal, and his dealings with Charles Keating, an Arizona high-flier whose failed thrift was a $2 billion debacle for taxpayers, were dominating the news in his home state.

Mr. McCain faced a hail of hostile questions, even at the Mesa Rotary Club in bedrock Republican territory. After one of the club's elders rose to defend him and say he still believed in him, the senator mordantly observed, ''The fact that my supporters even feel they need to say these things is evidence of how serious the situation is.''

''It was certainly the most difficult experience in my political life,'' he now says about this tumultuous time. During a 14-month Senate ethics investigation that ended with his exoneration, he and four other senators stood accused of exerting improper influence by meeting with federal bank regulators on behalf of Mr. Keating, who had contributed $1.5 million to the political causes and campaigns of the senators, including $112,000 to Mr. McCain.

Mr. McCain told allies, including his lawyer, John W. Dowd; a former Senate colleague, Warren Rudman; and a Senate aide, that the Keating investigation was more painful than being held as a prisoner of war in North Vietnam.

''It was a searing experience for John,'' said Mr. Dowd, who represented him during the ethics inquiry. ''He told me it was worse than being in Hanoi.''

The ordeal left a stain on his Senate record, and some political strategists believe the issue will come up again if Mr. McCain continues to gain traction in a presidential bid in which he has been highlighting his crusade against special interests.

But the Keating mess also stoked and hardened Mr. McCain's determination to be his own man in the Senate, to go his own way no matter what. These McCain traits are appealing to many voters, especially in states like New Hampshire.

His time under ethics scrutiny ''only further encouraged him to follow his own compass,'' said Scott Celley, his former press secretary. ''His attitude was that 'I've got to decide what's right for myself and go at it without relenting, no matter who is standing in the way.' '' Mr. Celley says Senator McCain has carried that attitude with him in other fights, from overhauling campaign finance to federal anti-tobacco legislation to this year's budget battle, in which the Arizona Republican has denounced pork-barrel spending.

The cloud of scandal did not lift until Nov. 20, 1991, when he was exonerated by the Senate Ethics Committee after an investigation that included weeks of televised hearings. The committee found that Mr. McCain had ''exercised poor judgment'' in attending, with four other senators, two meetings with federal banking regulators, but it found no improper action on his part.

''I found nothing in my investigation which caused me to question Senator McCain's integrity,'' said Robert Bennett, the Washington lawyer who was special counsel to the ethics panel during what is called the Keating Five investigation. Mr. Bennett, more recently, has been one of President Clinton's lawyers.

Senator McCain had taken $112,000 in Keating-related campaign donations, trips aboard Mr. Keating's corporate jet and family vacations at the executive's Bahamas hideaway. While legal, these gifts made his attendance at the meetings with federal regulators all the more questionable. (The other four senators had also taken large contributions from Mr. Keating, some of them far more than Mr. McCain.)

He survived and was re-elected easily in 1992 and again, with almost 70 percent of his state's vote, in 1998. Three other members of the Keating Five were more seriously rebuked by the ethics panel and all of them retired rather than face difficult re-election battles. John Glenn of Ohio, who was exonerated with Mr. McCain, was also re-elected in 1992, but retired in 1998.

Now that he is running for president, and with the passage of time, Mr. McCain says he does not view the Keating Five investigation as a ''defining moment'' in his career, though many of his friends and other political observers say it did influence his development as a crusader for changing campaign finance and as a Senate maverick. He also now says that the experiences of going through five and a half years as a prisoner of war and the Keating investigation are not comparable.