There's a big difference between the Republican and Democratic campaigns: The Republicans have split on policy grounds; the Democrats haven't. There's been a Republican divide between center and right but no Democratic divide between center and left.
When you think about it, the Democratic policy unity is a mirage. If the Democrats actually win the White House, the tensions would resurface with a vengeance.
The first big rift would involve Iraq. Both Sens. Hillary Clinton and Barack Obama have seductively hinted that they would withdraw almost all U.S. troops within 12 to 16 months. But if either of them did that, he or she would instantly make Iraq the consuming partisan fight of the presidency.
There would be private but powerful opposition from Arab leaders, who would fear a return to 2006 chaos. There would be irate opposition from important sections of the military, who would feel that the United States was squandering the gains of the previous year. A Democratic president with few military credentials would confront outraged and highly photogenic colonels screaming betrayal.
There would be important criticism from nonpartisan military experts. In his latest report, the much-cited Anthony Cordesman describes an improving Iraqi security situation that still requires "strategic patience" and another five years to become self-sustaining.
There would be furious opposition from Republicans and many independents. They would argue that you can't evacuate troops just as Iraqis are about to hold national elections and tensions are at their highest. They would point out that it's insanity to end local reconstruction and Iraqi training just when they are producing results. They would accuse the new administration of reverse-Rumsfeldism, of ignoring post-surge realities and of imposing an ideological solution on a complex situation.
All dreams of changing the tone in Washington would be gone. All of Obama's unity hopes would evaporate. And if the situation did deteriorate after a quick withdrawal, as the National Intelligence Estimate warns, the bloodshed would be on the new president's head.
Therefore, when a new Democratic administration considered all these possibilities, its members would part ways. A certain number of centrists would conclude that rapid withdrawal is a mistake. They would say that the situation had changed and would call for a strategic review. They'd recommend a long, slow, conditions-based withdrawal -- constant, small troop reductions, and a lot of regional diplomacy, while maintaining tens of thousands of troops in Iraq for the remainder of the term.