Progressives: we've forgotten (or maybe just never learned) the ideas of "critical support" and "strategic voting" which European center-left voters have applied for years. It's how they defeated conservatives many times in the post-war period.
One reason for the low enthusiasm among many Obama voters is their feeling that voting for Democrats who have been vacillating or inconsistent in their support for a robust progressive-Democratic agenda means those politicians completely get away with "taking progressive votes for granted" or "betraying progressive supporters"
From this point of view, the only way progressives can ever really have any influence on "Blue Dog" and other centrist Democrats is to "punish" them by staying home on Election Day.
In Europe, the voters who are the equivalent of American liberals and progressives have never thought about politics this way. Because the European center-left outside of Britain has historically been divided into several center-left parties rather than one umbrella party like the Democrats the voters realized that - if they ever wanted to build a majority coalition --they had to agree - after voting for their own preferred party in a first round of elections - to support the candidate of whichever coalition-partner got the most votes in a second round of voting.
These voters did not feel "betrayed" or "taken for granted" because even as more left-wing voters in some districts had to support candidates to their right, centrist voters in other districts supported candidates to their left. Both sides understood that their common interests would be better served by cooperating than by their acting alone.
Now I can already hear U.S. progressives complain "Yeah, sure, but here in America we're always the ones who have to make all the compromises and never the Blue Dog types", "We're always the ones who have to take it on the chin", "it's always a one-way deal"
Except that it's not. During the 50's and 60's, in dozens and dozens of congressional districts blue- collar Democrats loyally voted for Democratic candidates who were much more liberal than they were on social issues. They did it out of a combination of party loyalty and trust that the Democratic candidate would be more pro-labor on economic issues.
I mean, come on. Did you really think all those Irish guys sitting around the taverns in Southie for the last 40 years were just peachy-keen thrilled with Teddy Kennedy's position on about 500 different liberal social issues or that the guys in Al Gore's old district were slapping "save the whales" bumper stickers on their pickups?. No, they were doing a New Deal version of "critical support" and "strategic voting".
And meanwhile guess who was "taking them for granted" year after year - yeah, that's right, progressives. Go back and look at how many liberal commentators said Kennedy's senate seat was clearly a lock because it was "Teddy's old seat" and those blue-collar guys would never vote for a Republican.
So come on progressives, let's set aside the "we're the only ones who ever, ever, have to compromise" rationalization and start thinking strategically about the coming election.
If we want Nancy Pelosi to keep being the Speaker of the House, it ain't gonna happen because we sit at home and stew because the Democratic candidate in our district is too conservative for our tastes. You vote for whichever Democrat won the primary because that's how you support Nancy Pelosi and Alan Grayson and Al Franken and all the other Democrats who you do like. That's the meaning of strategic voting and critical support.