Actually, Kerry's 48.3% in 2004
https://en.wikipedia.org/wiki/United_States_presidential_election,_2004 was virtually identical with Gore's 48.4% in 2000.
https://en.wikipedia.org/wiki/United_States_presidential_election,_2000 Anyway, there have thus far only been five presidential elections since 1996, and it is rather arbitrary to assume that such a small number of elections can set a minimum or maximum percentage for a party. In any event, there is no particular reason to think Gore would be a better campaigner than Kerry--after all, he lost in 2000, despite peace and prosperity, and he did poorly in the second debate according to Gallup.
http://www.gallup.com/poll/2443/instant-reaction-bush-beats-gore-second-debate.aspx
Of course Gore or almost any other Democrat could win in 2008, but that is another matter. 2004 was simply not as unfavorable a year for the GOP in terms of underlying circumstances as some people seem to think. The economy was improving. Bush's huge bump from 9/11 had largely been eroded, but the GOP was still seen as the party that was "tougher" on terrorism. The war in Iraq was not nearly as popular as it had been in 2003 but it also wasn't nearly as unpopular as it would become by 2006. All in all, a fairly narrow Bush victory is exactly what one would expect (most presidents do get re-elected, after all) and IMO Kerry neither underperformed nor overperformed. I just don't see Gore doing so much better than Kerry in Ohio as to overcome Kerry's over-two-point loss there; after all, Gore had lost Ohio by over three points in 2000....