Obama enjoys running for president, but he doesn't want the responsibilities that go with it. Romney is just the opposite. He's interested in the office, but won't make any real effort in getting it in the first place.
You are right when it comes to running against liberals. Romney won’t put any effort into it at all. But when it comes to defeating conservatives he pulls out all the stops. I had never seen anything like the ferocity he used going after Newt Gingrich, but just a few months later in the same state of Florida against Obama, nothing. Never seen anything like that either. Going up and down the state telling people how much you like your opponent is not much of a campaign.