More than any other issue, Sens. Barack Obama (D-Ill.) and John McCain (R-Ariz.) owe their nominations for president to the war in Iraq. One man foretold disaster in his early opposition; the other bravely backed the correct strategy for reducing bloodshed. Both candidates chose the lonely path — Obama among Democrats supporting the war in 2002 and McCain among Republicans jumping ship five years later.

When Obama and McCain officially began their quests for the White House, opposition to the war was at its peak. Yet as chaos and violence brought clarity to Americans, when McCain’s candidacy appeared doomed, a more stable Iraq has now sown doubt among voters who worry about the consequences of withdrawal, which Obama has planned within 16 months of taking office. This new uncertainty, about an unpopular $10 billion-per-month war a majority of Americans believe should not have been waged, strikes at the heart of the Obama candidacy and distinctly benefits McCain. Moreover, by embracing Obama’s call for vigilance in Afghanistan and Pakistan, McCain is cornering him, highlighting further their differences on Iraq.

While polls once showed steady support for withdrawal, a new ABC News survey shows it has dwindled to just 50-49. Even worse for Obama is strong support favoring McCain as commander in chief by 72-48.

When Obama recently announced his plans to visit Iraq, saying he could possibly “refine” his policy after consulting with commanders there, Republicans and the media pounced. Pushing back against charges of flip-flopping on the issue that helped him vanquish Sen. Hillary Rodham Clinton (D-N.Y.), Obama is now reiterating his call to end the war.

Most Americans would agree with many of the things Obama said in his major foreign policy address this week. The situation in Afghanistan is worse, June saw the most casualties there, the Taliban is emboldened and al Qaeda finds sanctuary in Pakistan. Check. Obama said he warned “the invasion of a country that posed no imminent threat would fan the flames of extremism, and distract us from the fight against al Qaeda and the Taliban; Sen. McCain said we would be greeted as liberators, and that democracy would spread across the Middle East. Those were the judgments we made on the most important strategic question since the end of the Cold War.” Check.

Obama’s speech was smart, thoroughly detailed and ambitious. In it he quoted George Marshall, who upon announcing the Marshall Plan said, “The whole world of the future hangs on a proper judgment.” Yet while touting his own judgment and criticizing the war, Obama also validated McCain’s claim that he has been proven right when Obama acknowledged the surge’s success in lowering violence, saying, “Those are the facts, and all Americans welcome them.” Obama has also, according to press reports, removed criticism of the surge from his website.

When Obama returns from Iraq, he will take what must be his final stand on the war for the rest of the campaign. He won’t be able to say what McCain did this week: “I know how to win wars. And if I’m elected president, I will turn around the war in Afghanistan, just as we have turned around the war in Iraq, with a comprehensive strategy for victory.”

While McCain’s words aren’t necessarily reassuring, whatever Obama says next isn’t likely to bring back the certainty Americans felt about the war 18 months ago. Obama is hoping Americans will choose his judgment over McCain’s experience, even if it rocks the boat again in Iraq. But he is also probably hoping the election won’t be decided on Iraq. If it is, it could be checkmate.

Stoddard is an associate editor of The Hill.