By historic standards, 2004 was an extraordinarily close election. Given the election's closeness, it is incredible that John Kerry won a smaller percentage (6%) of Republican voters than Walter Mondale won in the blow-out Reagan landslide of 1984.
In addition to the turn-out of Republicans, this Republican loyalty to Bush may have been the key determinant of the election since Kerry won independents and did well among Democrats. (In fact, 2004 was the first election in which the Democratic candidate won a plurality of independent voters and lost the election.)
http://www.cnn.com/ELECTION/2004/pages/results/states/US/P/00/epolls.0.html (scroll down to Vote by Party ID)
http://www.ropercenter.uconn.edu/elect_2004/voted_84.html (scroll down to Party)
See below for my explanation.
Republican loyalty to Bush is also extraordinary when one considers Bush's poor and controversial performance in office and the doubts about his policies that existed even among Republicans. I believe the reason for the strength of Bush's support among Republican voters can be attributed primarily to the size and power of the Republican megaphone and its associated echo chamber on talk radio, cable television, conservative churches and local television news and newspapers outside of the largest metropolitan areas. This enabled the Republican propaganda machine to overcome doubts about Bush among Republicans by creating and continually repeating dubious claims about Kerry's character, ideology and policies. By election day, most Republicans believed that whatever their concerns about Bush, Kerry would be far worse.
My guess is that 10-15% of Republicans had sufficient doubts about Bush that the Democrats could have exploited without altering their policy positions had they been more adroit in amplifying these doubts. In addition to media influences, I think almost all Congressional Democrats deserve blame for playing patty-cake with Bush until the Fall of 2003. And for continuing this game through 2004, Me-too Red State Democrats deserve particular blame for partially embracing Bush and being unwilling to take even moderate political risks to knock down Bush's standing. Republican politicians like Hagel, Lugar and McCain seemed more willing to take shots at Bush than Red State Democrats.