Contra Drum, Andrew Sprung believes Obama will paint Romney as both a flip-flopper and an extremist:

It seems to me … that the flip-flopper charge and the extremist charge are complementary.  Romney, in pursuit of a nomination prize awarded by the Republican base, has flip-flopped into extremism: no new taxes, no universal healthcare, no abortion, no attempts to limit greenhouse gas emissions, no aid to homeowners, no job-creating measures that involve federal spending, no accommodation for Gingrich-styled "deserving" undocumented aliens.  Each of these positions represents a reversal of a past position (except perhaps aid to hurting homeowners).