As demonstrated by the fierce White House counterattack in recent days on critics of the Iraq war, no one has more riding on the outcome of that war than President Bush, the man who sent U.S. forces into Baghdad.

But in political terms, the man next most affected by the outcome of the fighting could be Sen. John McCain of Arizona.

No one outside the administration has been more adamant or outspoken in arguing that there is no substitute for victory in Iraq than has McCain, the Naval Academy graduate and survivor of years in a North Vietnamese prison camp. Others in the field of potential 2008 presidential candidates also support the war, but for none of them does it represent as large a gamble.

McCain's unique credential as a presidential candidate is his hard-earned reputation as someone who rises above partisanship. While burnishing his lifelong Republican credentials by his support of Bush in two campaigns, McCain has established himself as the favorite of independents in poll after poll while enjoying the approval of many Democrats for his advocacy of governmental reforms.

Unscarred in his psyche by the wounds that the 1960s and '70s left on a whole generation of baby-boomer politicians, McCain, who was born in 1936, a full decade before the earliest of the boomers, is a throwback to an earlier generation of leaders who recognized the value of building partnerships across party lines.

He has genuine friendships with Democratic colleagues, and his life is marked by successful efforts at personal reconciliation with people who have been on the opposite side of important policy debates.

Amid signs that the voters are sick of excessive partisanship and looking for a leader who really is, as Bush claimed to be, "a uniter, not a divider," McCain has surged to the top of any list of potential 2008 candidates.

But there is nothing nuanced about his position on the Iraq war. In speeches on and off the Senate floor and in countless television interviews, McCain has argued that it was right to remove Saddam Hussein and that the United States and its allies must remain in Iraq until conditions are created for a stable, secure Iraqi government.

When I interviewed him in his office the other day, he even used the pejorative phrase "cut and run" to describe those now calling for a timetable for withdrawal of American troops. Time and again, he argued that the consequences of leaving Iraq prematurely would be a factional or religious struggle within that country that could lead to a radical Islamic regime destabilizing the Middle East and threatening more terrorist attacks.

The striking thing about McCain's position, which has not wavered from the beginning of the debate about going to war, is that no one has been more critical of the conduct of the war than the senator from Arizona.

As he reminded me, when he made his first trip to Iraq after the capture of Baghdad, he encountered a dozen junior officers of the American and British forces who told him in vivid terms how they were hampered by the shortage of troops. At breakfast with Defense Secretary Donald Rumsfeld after he returned, he urged Rumsfeld to bolster the manpower in Iraq, only to be told, "The generals are not asking for more troops" -- as if, McCain added scornfully, "any commander is ever going to make that kind of request."

The misjudgments, McCain said, have continued down to the present. He could not believe, he said, that Rumsfeld pulled Lt. Gen. David Petraeus, the man who finally organized the first training program for the Iraqi army to show some positive results, out of Iraq this summer for a prestigious but hardly vital assignment at Fort Leavenworth, Kan.

When I saw McCain, he had not yet read James Fallows's cover story in the December Atlantic magazine, titled "Why Iraq Has No Army." In an amply documented and deeply disturbing account, Fallows shows how hollow has been the administration claim to "standing up" Iraqi security forces capable of replacing the U.S. troops. Fallows also argues that doing so at this point would require fundamental shifts in Pentagon priorities -- on everything from troop rotation to the allocation of weapons budgets -- that are not likely to come from Rumsfeld or Bush.

Much of McCain's critique of the management of the war is echoed in Fallows's argument. Nonetheless, McCain insists that victory is still possible -- and that it is vital. Majorities of both independents and Democrats now say the war was a mistake. McCain disagrees. As is his custom, he seems perfectly willing to rest his political future on his belief in his own principles.