By DAVID D. KIRKPATRICK

Friends of Senator John McCain say there is an essential tension that runs through his life. He is often deliberative, self-critical and flexible, his advisers and fellow senators say, and has frequently corrected course during his 36 years in public life.
“He is a much more supple mind than he is usually portrayed,” said Philip Bobbitt, an international relations scholar and Democrat the senator consulted this summer.
But when he confronts an adversary, a starkly different John McCain can emerge, fired up with certainty for an all-or-nothing battle. “I am going to win this thing and you are going to have to run me over to defeat me,” said former Senator Bob Kerrey, a Democrat close to Mr. McCain, explaining his friend’s attitude.
The conflicting impulses toward deliberation and aggression have been the alternating currents of his singular career and, if Mr. McCain wins the White House, could shape his presidency.
In the Senate, he is almost as well known for his handwritten apology notes as for his outbursts. (“I think I learned a few things in prison but possibly one of the most important things was the value of friendship,” Mr. McCain wrote in one note provided to The Times. “Chalk it up to the ‘McCain temper.’”)
He fires advisers who disappoint or embarrass him, but then keeps seeking their advice. He frets publicly that his ambition might tempt him to compromise his principles, but he also races headlong into battles in pursuit of political power.
If elected on Tuesday, which polls say is unlikely, Mr. McCain would arrive well-scarred at the White House: 72 years old, the oldest president to enter office, the first Vietnam veteran, a survivor of five and a half years in a North Vietnamese prison camp .
Driven as much by his notion of honor as by ideology, Mr. McCain could make an unpredictable - his critics say “erratic” - chief executive. By default he is a limitedgovernment conservative, but he readily bends those convictions if a cause seems worthy. He has regularly picked fights with both parties , but also knows how to force through bipartisan deals.
Mr. McCain has called his decision-making style “instinctive, often impulsive,” as he put it in “Worth the Fighting For,” a 2002 memoir written with his aide Mark Salter. “I don’t torture myself over decisions. I make them as quickly as I can, quicker than the other fellow if I can.”
He first tasted politics in 1977 as the Navy’s liaison to the United States Senate. He was 40 and unsure of his future, and he turned the assignment into a training seminar for his own political career. Escorting lawmakers on overseas trips and entertaining them with stories of his naval escapades, Mr. McCain listened as the senators gossiped over evening cocktails, or brought him into closed committee staff meetings. And he capitalized on their goodwill: Senator William Cohen of Maine, best man at Mr. McCain’s 1980 wedding, and Senator John Tower of Texas, both Republicans, provided invaluable help in his 1982 election to a House seat in Arizona.
As a senator or presidential candidate, Mr. McCain prefers to make decisions by consulting experts with opposing views, preferably watching them clash. “He encourages disagreement in front of him, to see the evidence that disagrees with where he might be headed,” said Kevin A. Hassett, an economist close to Mr. McCain.
He can take defeat hard. After conservatives blocked a major tobacco bill he had negotiated in 1998, Mr. McCain excoriated his own party for consigning children to lung cancer. After losing fights over campaign finance rules, he would lash out at his opponents as corrupt.
He relishes conflict, his friends say, and would make a confrontational president. As Senator Lindsey Graham, Republican of South Carolina and a close friend put it: “The man will run across the street to get in a good fight.”
In more reflective moments, Mr. McCain says he tries to maintain a stoic detachment about the prospect of victory or defeat, a habit of mind he says he acquired as a Navy pilot and prisoner of war. “I tend to be fatalistic about these things,” he said in an interview not long after he had locked up the Republican nomination, shrugging off his success.
Contemplating his 2000 run at the White House, he worried about balancing his ambition for the prize with his own sense of virtue, he wrote in “Worth the Fighting For.”
After his loss, he professed himself grateful, at the age of 65, for what might be left of his time. “I did not get to be president of the United States. And I doubt I shall have reason or opportunity to try again,” he wrote, but added, “I might yet become the man I always wanted to be.”