Kerry's primary was over much quicker. New Hampshire basically ended it. Obviously the challenger will strengthen now that the general started yesterday.
Romney was already in near-record unfavorability territory by the end of January, so you can't entirely blame the primary for that.
Actually, Romney's the one who should be trying to take a page from Clinton's book -- a challenger running against an incumbent in tough economic times, whose unfavorability provides the only precedent for Romney's. Of course, Bill Clinton was unpopular because he slept around, and won the election by feeling people's pain. Romney's unpopular because he's rich and aloof and awkward. "Understanding the problems of people like you" is Romney's Achilles heel.