As should be obvious to anybody, especially to anybody who reads Mudcat..

1)   Liberals, not centrists, are the foot-soldiers of virtually any Democratic campaign.
Consider who the darling of the most vocal Democrats was. Was Dean any less liberal than Kerry? Doubtful.

2)   Primary system, as set up, benefits the true believers in each party (who are also the above-cited foot soldiers.)--who are on the left for the Democrats, the right for the Republicans.