WASHINGTON, D.C. -- I asked one of the Republican Party's smartest, most candid heavy hitters this week whether John McCain really has a chance to defeat Barack Obama in this season of Republican discontent. "No, if the campaign is about McCain," he replied. "Yes, if it's about Obama." That underlines the importance of Obama's visit to Iraq, beginning weeks of scrutiny for the Democratic presidential candidate under a GOP spotlight.

Four years ago nearly to the day, I asked the same question to the same Republican leader about George W. Bush and John Kerry, and he gave the same answer. He proved prophetic because Bush's campaign made Kerry the issue, and the 2004 Democratic presidential candidate flunked the test.

Obama is a far more interesting personality and an incomparably more appealing candidate than Kerry. But why then, in a year where the nation clearly has rejected the GOP as a party, does McCain have a real chance to be elected? Why does Obama have trouble breaking the 50 percent barrier, nationally and in battleground states?

The answer, as seen by McCain's closest associates, is the issue they hope to ride to victory: leadership. They believe voters are hesitant to fully accept this charismatic newcomer because of doubt as to whether he can lead the nation. Now, in visiting Iraq for the first time in two and a half years, Obama tests that issue. In what on the surface looms as a public relations coup for Obama, the McCain camp will be scrutinizing -- and commenting on -- his every move in Iraq.

Obama may have been goaded into visiting the war zone by taunts from Sen. Lindsey Graham, McCain's friend and adviser, that the Democratic candidate had not been to Iraq since January 2006. But once he decided on going to Europe, Afghanistan and Iraq, Obama had Republican loyalists worried sick. Predictably greeted as a conquering hero by Bush-hating Europeans, a champion by apprehensive Afghans and a liberator by war-weary Iraqis (with massive media coverage), Obama may get the big bounce in the polls that eluded him when he clinched the nomination against Hillary Clinton.

Nevertheless, Obama in Iraq spotlights the question that McCain wants asked: Who can best lead America in a dangerous world? The difficulty posed for Obama by the leadership issue was demonstrated last week, when he preceded his fact-finding mission with a speech pronouncing that he has not really modified the hard anti-war line he used to defeat Clinton for the nomination. (In private conversations, Clinton has expressed the view that Obama's emphasis on Iraq -- her Senate vote for it, his against it -- defeated her.)

Since clinching the nomination, Obama has been cautiously executing a Nixonian post-primary pivot toward the center. He weathered outrage by his "net-roots" bloggers over his vote for the national security wiretapping bill. But hedging on Iraq was vastly more dangerous, particularly when it appeared he was modifying his famous pledge to remove U.S. troops within 16 months after becoming president.

So, in his pre-trip speech last Tuesday, he reaffirmed the 16-month deadline (though in less robust style than on the primary election circuit): "We can safely redeploy our combat brigades at a pace that would remove them in 16 months." But he added, cryptically, "We'll keep a residual force" for "targeting any remnants of al-Qaida," "protecting" remaining U.S. troops and officials, and training Iraqi security forces provided they "make political progress."

How big would this more or less permanent "residual" force be? Obama did not say, but advisers leaked it could reach 50,000. That would be far too much for the candidate's net-roots to swallow, but a token force of around 2,000 would be ludicrous. Obama will face a test of how he handles this after he meets in Iraq with the esteemed Gen. David Petraeus.

Obama's speech continued his campaign's theme of depicting a McCain administration as Bush's third term, in this instance continuing present Iraq policy. But the spotlight of scrutiny will be on Obama, not McCain, because of his decision to visit Iraq, and therein lies McCain's hope for victory.

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In the last column, I misidentified James Johnson as having been at ZymoGenetics Inc. That is a different James Johnson. It correctly identified Johnson as heading the compensation committee at Goldman Sachs, which was the point of the column.