The shadow of the Iraq War still hovers over the 2008 presidential race. In deed, though it's the issue that made Barack Obama (giving him his running room to Hillary Clinton's left), it may now become his chief vulnerability.
Weak on national-security issues, untried, inexperienced and (perhaps) naive, Obama can find the Iraq issue hard to handle - if John McCain plays it right.
Obama has long since won the issue of Iraq-past - opposing the war before anyone and voting continuously and solidly against it when others waffled.
Yet McCain is winning Iraq-present: A majority of Americans believe that the surge is working. Casualties are down so far that the pessimistic left has shifted its doom-and-gloom to Afghanistan.
But McCain's key opportunity is to exploit the issue of Iraq-future.
To start, he must ask Obama: "Why won't your troop withdrawal allow al Qaeda and Iran to move into the vacuum, taking over Iraq to use it as a base for terror against us and Israel?"
Obama will hem and haw, but McCain must keep at him - and force his opponent to confront the consequences.
How will Obama answer?
He can't shift his position on his signature issue much more - or he'd get an even worse rap for flip-flopping. So he'll start by stressing the ongoing troop presence that he'll allow in Iraq.
He has said (vaguely) that he'll permit sufficient troops to cover our pullout, protect our embassy and pursue al Qaeda terrorists. Now he'll try to sell the idea that his gradual withdrawal over 16 months and his ongoing troop commitment will hold al Qaeda and Iran at bay.
But who'll believe that? Experience has taught Americans to expect the worst about Iraq. They're inclined to agree that, if we pull out, al Qaeda will move in. It's also self-evident that Iranian influence will grow as ours' declines. (To the extent that we do believe it, Obama will alienate the left and drive voters to Ralph Nader.)
His next dodge will be to talk up diplomacy - that a dialogue with the mullahs can hold Iran at bay. But no negotiations are possible with al Qaeda - and Americans realize that talks with Iran will go nowhere unless we have the leverage of force. His reliance on diplomacy will come off as naive, reinforcing the impression that he's not ready for the job.
Eventually, he'll have to say he's prepared to go back into Iraq if the situation deteriorates. Voters will realize that a nominal troop presence and diplomacy won't do the job.
That's when McCain moves in for the kill: "So, isn't your rigid adherence to withdrawal inviting a third Iraq War?"