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Despite the more-sarcastic-than-usual tone, Glenn makes a good point. Progressives have been largely fooled into being reflexively loyal to the Democratic Party -- to a similar or greater extent than conservatives have been fooled into being reflexively loyal to the Republican Party.
The Democratic Party does have some progressive tendencies but it is an Establishment institution, heavily entrenched in DC and the state capitals. The Party and its leaders care little for the progressives that makes its base; yes, they'll throw us bones from time to time like an investigation or two into some particularly heinous abuse of the Bush Administration.
But when push comes to shove and the Democratic Party has to choose between allowing the government to retain/seize more power or upholding progressive values, the Democratic Party will invariably choose power. Even if that means forming an unholy alliance with the Republicans because the Democratic Party knows that someday that power will be theirs, as well.
Furthermore, the highly partisan nature of today's electorate makes it even easier for the Democratic Party to choose power. Since every proposition will automatically come down on one side of the Blue/Red divide, at least 40% of the electorate will always view every idea as "bad for its side" meaning that it is nearly impossible for there to be a critical mass of people to force even the most reasonable reforms.