Obama is trying to "define" Romney by showing he is not "one of us." It is clear that Romney's track record of going negative in every priary means he'll try to go negative against Obama. In 2008 Obama and Hillary went at it pretty heavily and sometimes negatively, and in the general election Obama was running on "hope and change" which he cannot do this time. Additionally Sarah Palin aside McCain ran a reasonably clean campaign so neither side really went that negative.