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 actually 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 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.

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 postsurge 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.

The left wing of the party would go into immediate uproar. They'd scream: This was a central issue of the campaign! All the troops must get out now!

The president would have to make a terrible decision.

Which brings us to second looming Democratic divide: domestic spending. Both campaigns promise fiscal discipline, as well as ambitious new programs. These kinds of have-your-cake-and-eat-it-too vows were merely laughable last year when the federal deficit was running at a manageable $163 billion a year. But the economic slowdown, the hangover from the Bush years and the growing bite of entitlements mean that the federal deficit will almost certainly top $400 billion by 2009. The accumulated national debt will be in shouting distance of the $10 trillion mark. With that much red ink, the primary-season spending plans are simply ridiculous.

It'd be 1993 all over again. The new Democratic president would be faced with Bill Clinton's Robert Rubin vs. Robert Reich choice: either scale back priorities for the sake of fiscal discipline or blow through all known deficit records for the sake of bigger programs. Choose the former, and the new president would further outrage the left. Choose the latter and lose the financial establishment and the political center.

This is the debate that Democrats have been quietly rearguing during the entire Bush presidency. The left wing of the party is absolutely committed to winning it this time. It will likely demand the clean energy subsidies and the education spending, the expensive health care coverage and subsides to address middle-class anxiety. But no Democratic president can afford to offend independent voters with runaway spending. No president can easily ignore the think tank establishment, which is rightfully exercised about the nation's long-term fiscal health.

It would be another brutal choice.

As William J. Stuntz of Harvard Law School wrote in The Weekly Standard, the Democrats have conducted their race amid unconstrained "Yes We Can!" unreality. Because the Democratic candidates appear to agree on so much, they've never tested each other's policy proposals or exposed each other' assumptions. But governing means choosing, and reality will be unkind. The artificial unity between the Democratic center and the Democratic left would be smashed by the harsh choices of 2009. My guess? The centrists would win.

  c.2008 New York Times News Service