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, yet no Democratic divide between center and left.

But 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 Senators 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 actually did that, he or she would instantly make Iraq the consuming partisan fight of their 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 U.S. was squandering the gains of the previous year. A Democratic president with few military credentials would confront outraged and highly photogenic colonels screaming betrayal.