Hmmm, how about this.
Kerrey doesn't win the Democratic nomination, rather Howard Dean is able to avoid having his campaign implode. Dean, after winning Iowa, captures NH and cruises to victory in the primary. However, he is quickly portrayed as a extreme liberal by Karl Rove and the right, and he sees his poll numbers dip badly. In desperation he decides to try and select a conservative democrat to be his running mate. He considers Joe Lieberman, Wesley Clark, and John Edwards before taking a huge gamble: rather than select a conservative democrat he would select a liberal republican (in OTL John McCain considered this strategy when he entertained selecting Joe Lieberman as his running mate in 2008).
Dean is unable to find a republican who qualifies from the south, but one governor from New England seems to fit the bill. His advisors feel that this will alienate democrats and will not balance the ticket since both candidates are from New England. But the pick does boost his poll numbers (although Dean-Romney still looses to Bush-Cheney). However, Mitt Romney's acceptance of the VP spot on the Dean ticket ends his political career as a Republican. In 2006 he is defeated in the Republican Primary in Massachusetts despite having a 60% approval rating in the State. He runs (and wins) reelection as an independent (note, I am not sure if MA allows this, but most states do. See Florida). After serving his second term Romney announces he is switching parties to Democrat. His critics call it pure political opportunism but he still remains popular in Massachusetts, particularly since he starts to moderate some of his positions. In 2008 John McCain-Joe Lieberman defeat the ticket of Hillary Clinton- John Edwards when Edwards is revealed to have had an affair with a campaign aid, getting her pregnant in the process. However, McCain's term in office suffers from the blowback over the housing bubble crisis, and after a difficult primary against Rick Santorum in 2012 he agrees to drop Lieberman from the ticket and replace him with the conservative governor from Alaska: Sarah Palin. Palin proves to be a disaster and a huge liability to the campaign, as the McCain - Palin ticket starts to appear considerably more conservative than the McCain - Lieberman ticket that barely won in 2008 against an imploding Clinton - Edwards campaign. He ends up losing to the former republican governor of Massachusetts, Mitt Romney, who selected Illinois Senator Barak Obama as his running mate.