At the moment, though, there’s a big difference between the two parties’ divisions: The Democrats’ fault lines are primarily demographic (upscale vs. downscale, professional vs. working class, women vs. men), whereas the GOP’s fault lines are demographic and ideological.
This is right, but it’s also important to note that the fault lines within the Democratic Party seem to be mostly persistent and enduring ones. The same divide seems to keep replaying over and over in every cycle, and as we all know Democrats have usually ended up choosing the constituency-oriented incumbent or machine politician rather than the upscale candidate of “new ideas” or reform. I would add that there was a combination of demographic and ideological fault lines in the 2004 Democratic race, to the extent that there was a relatively more progressive, netroots-backed antiwar candidate of a sort running against a relatively more hawkish liberal and the then-”centrist” Edwards. The ideological divide was really more between the supporters of the candidates than between the candidates themselves, and the election exaggerated the extent of this divide, but it was there.
The main contenders and the eventual nominees on the ticket in ’04 arguably represented the last hurrah of neoliberalism on the Democratic side, and the last three years have seen the gradual strengthening of progressives within the party to the point where all three leading Democrats are running on a platform as progressive as any there has been in my lifetime and probably more so. Ross is also right to point out that the prospect of victory and the desire to capture the White House are uniting the Demmocrats. Just as the hunger to win and eight years out of executive power pushed Republicans to unite around Bush in 2000, the Democrats are suppressing whatever real ideological arguments they might have over foreign policy, trade or anything else for the sake of winning. In the ideological fragmentation on the GOP side, we are seeing something like the Republicans’ 1968 moment, but to the general convulsions within the party there is the added problem that there is also no incumbent to lead the party, which exaggerates the effect of the disagreements. It makes the nomination fight a contest over the future direction of the party in a way that 2004 did not affect the Democrats.