WASHINGTON -- It is a political error for a candidate to believe that voters who agree with him will always end up supporting him.

There is little doubt that Americans generally feel that the initial use of military force in Iraq was a mistake. Recent, paradoxical polls show a dramatic increase in the number of people who believe that the war is now going well alongside a hardening majority who believe it should not have been begun at all. Barack Obama's strongest argument on Iraq is increasingly about the past.

But presidential elections tend to focus on the future. In spite of their past failures, whom do you trust more to conduct a flawed, messy war in the years ahead? Lincoln or McClellan? Nixon or McGovern? Bush or Kerry? McCain or Obama?

At some point, most foreign policy debates, especially during a war, come down to a binary determination: Is a candidate strong or weak? Voters can disagree with a nominee on many things and still find him stronger than his opponent.

So far, Obama has not taken this challenge with sufficient seriousness. His Iraq approach comes down to three points. First, he has voted twice against funding U.S. troops in the field -- a political necessity in the Democratic primaries, but a blunder with the broader electorate. No matter what subtleties Obama attempts to develop in his Iraq position, this will be seen as a symbol of impulsive radicalism, unbecoming in a commander in chief.

Second, Obama advocates a specific timetable for the withdrawal of American combat troops in order to pressure the Iraqi government to take its responsibilities more seriously. (In fact, according to Obama's January 2007 Iraq plan, all combat troops would already be out of Iraq.) But it seems increasingly unfair to denigrate the efforts of Prime Minister Nouri al-Maliki's government, which has moved forward on 12 of 18 benchmarks set by Congress, and has recently engaged Shiite militias in a fight the U.S. has been demanding. In many cases, the Iraqis seem to lack capacity, not will -- which is precisely Gen. David Petraeus' argument for continued American engagement.

Third, Obama promises to personally negotiate with President Mahmoud Ahmadinejad on Iran's destabilizing support and training of Shiite militias. What might seem a bold strategic maneuver from a Nixon or Kissinger smacks of dangerous naivete from a fourth-year senator.

Obama -- the most reflective of candidates -- displays little self-knowledge when it comes to these political challenges. When questioned recently about his choice for vice president, he responded, "I would like somebody who knows about a bunch of stuff that I am not as expert on. I think a lot of people assume that might be some sort of military thing to make me look more commander in chief-like. ... Ironically, this is an area -- foreign policy is the area where I am probably most confident that I know more and understand the world better than Senator Clinton or Senator McCain."

The question here is not self-confidence but public confidence. And Obama's political judgment is exactly wrong. He will have enormous advantages on domestic policy in the coming campaign, on which he seems both more activist and interested than McCain. But McCain leads on measures such as "strong leader." Obama needs to seem, and be, more commander in chief-like.

McCain has challenges of his own. The fortunes of his campaign remain tied to events in Iraq, as they have been from the beginning. And despite undeniable progress against Sunni radicalism, events in Iraq are still inseparable from the actions and attitudes of Shiite militias armed and directed by Iran -- an influence that went unconfronted by America for many years. Maliki's uncoordinated attack on the Shiite militias in Basra seems to indicate that while the Iraqi spirit is willing, the flesh remains weak. But the failure of the Shiite uprising to spread more broadly shows that the extremists may be weaker than in the past. And, as Fred Kagan of the American Enterprise Institute points out, Moqtada al-Sadr was forced to cave in at the end. "By going after al-Sadr," he says, "Maliki forced the Iraqi political parties to take sides, and every single one sided with him (Maliki)."

The situation in Iraq, as Gen. Petraeus insists, is "fragile and reversible." But the debate has moved far beyond a candidate's initial support for the war. This has led to an odd inversion of the generational battle. Young Obama's strongest arguments are focused on the failures of the past. The older man, by insisting on victory, is more responsible and realistic about the future.