There is a four percent difference, favoring Romney, in the amount of 2008 Obama voters switching sides to say they will vote for Mitt Romney to the amount of 2008 McCain voters that say they will vote for Obama.
Via Gallup:
Eighty-six percent of voters who say they voted for Barack Obama in 2008 are backing Obama again this year, a smaller proportion than the 92% of 2008 John McCain voters who are supporting 2012 Republican candidate Mitt Romney. Nine percent of 2008 Obama voters have switched to supporting Romney this year, while 5% of McCain voters have switched to Obama.
In an election that most believe will be very close, that four percent advantage for Romney could be crucial.
More:
Nonwhites other than blacks, a group composed largely of Hispanics and Asians, are among the subgroups of 2008 Obama or McCain voters most likely to switch presidential preferences or be undecided this year. Twenty-one percent of this group of voters have a different preference now than in 2008, about double the national average of 11%.
Other groups showing an above-average shift in vote choice between 2008 and 2012 are political independents (18%), political moderates (16%), Eastern residents (15%), those with a high school education or less (15%), and unmarried men (15%).
With each of these demographics showing a drop of support for Obama since 2008, the fight for these voters will be brutal over the next three months. The negative campaigning, while moderately successful on the part of both candidates, is not going to be enough to swing those not fully decided yet on who they are going to vote for in November.
Saying "that guy is bad" will not be as effective as comparing why the candidate himself is at the same time as contrasting it to why 'that guy is bad."