2008 and the Time for a Change Factor
Democrats can't win by mobilizing the base alone--they need a candidate and a message with broader, at least slightly broader, appeal. Still, Kerry did not do badly for a Massachusetts liberal. This was no 1984 or even 1988. Only a 3 point margin in the popular vote and with another 150,000 votes in Ohio, Kerry would have won the electoral vote. (That's kind of a scary thought though. The electoral college could really misfire pretty badly, much worse than in 2000, and in either direction.)
2008 really should be a better chance for Dems to win the White House if we have a strong candidate and a strong message. Here's why--the time for change factor kicks in for us. And it's pretty big. Since WW II there have been 7 presidential elections in which a party had held the White House for just one term. The incumbent party's candidate won 6 out of 7 (only Carter lost) with an average popular vote margin of 11.6 percent. There have been 8 elections in which a party had held the White House for two terms or more. The incumbent party's candidate won only 2 and lost 6 with an average popular vote margin of -0.9 percent. This pattern goes all the way back to the beginning of the 20th century and the "time for change" factor has a significant effect on the outcomes of presidential elections even when you control for the incumbent president's popularity and the state of the economy--there is about a 5 point penalty if you've held the White House for 8 years or longer. Add that 5 points to Kerry's 2004 total, and you win easily.