What's interesting is the difficulty President Obama is likely to have in painting his opponent as an "extremist," if it turns out that Romney is the GOP nominee. In fact (as I noted here), the public sees the president as more ideologically out-of-step with them than they do Romney -- or any other GOP candidate, for that matter.
There are also warnings for a GOP nominee. It would be foolish to go after President Obama personally -- his ratings remain (relatively) high. This may reflect voters' reluctance to admit they were totally wrong about the guy, but whatever it is, it's worth noting.
The secret will be to keep the attacks impersonal, devoted to the price ordinary Americans have paid for the President's policy failures and the steps that need to be taken to undo the damage and start rebuilding. In the end, the most successful political campaigns are not about the challenger. They're not even about the incumbent. They're about the country -- and how best to unleash the inherent greatness of its people.
All that being said, the bottom line is this: This time around, the President is going to have to work hard to win the election.