A peculiarity of Barack Obama's reelection campaign is that the single most consequential thing he could do is get reelected. That's true even if, after he gets reelected, he isn't able to come to an agreement with Congress on anything more significant than keeping the lights on.
What makes Obama's most significant achievements unusual is that they roll out slowly. His key accomplishments were signed into law in his first term, but they won't be fully implemented by November. But if Obama is reelected, the Affordable Care Act will be implemented, on schedule, in 2014. At that point, it's likely permanent. The Dodd-Frank financial regulations will continue to be written and wrapped around Wall Street. At that point, they, too, are unlikely to be undone anytime soon.
Conversely, if Obama isn't reelected, both laws are likely to be fully or mostly repealed. And so the most lasting changes Obama has signed into law depend upon his reelection not just to survive, but to simply begin. But that's all they really need. They don't require another vote in Congress, or the buy-in of House Republicans. They just need to be left alone. They just need Barack Obama rather than Mitt Romney to be sitting in the Oval Office.
The same goes, in a way, for the tax code. If Obama is reelected, the Bush tax cuts expire on schedule, and they don't get extended unless he signs legislation extending them — which he has said he won't do, at least for the cuts for income over $250,000. In this case, it's cutting, not raising, taxes that requires compromise. So the question will be less whether Republicans want to work with Obama and more whether they want to bring down taxes, which they typically do.
Now, that's not to say there aren't a slew of policies Obama would like to pass and would need Republican cooperation to get done. A partial list would include the American Jobs Act, a major energy bill and immigration reform. But Ramesh Ponnuru is most likely right: There's little reason to believe that Republicans will be significantly more interested in compromising with Obama if he wins a second term.
What's unusual is that the policies that are most important to Obama and the Democrats don't actually require compromise with Republicans. Health care, financial regulation and tax increases are on autopilot. If Obama wins reelection, Americans are going to see a lot of change even if Republicans don't offer much cooperation. If he loses, much of the change he signed into law in his first term will never actually happen.