Eugene has a very well thought out and well argued diary on the rec list right now which explores what building a Progressive movement means and posits that it requires "moving on" from the goal of electing "any old Democrat." In considering the issue, eugene makes many good points (which I admit is as per usual even though I am usually arguing against him). Nonetheless, I think that it is possible to build a progressive movement without engaging and attacking the currently entrenched non-progressive elements of the party.
I would posit that the progressive majority may be more easily built by sweeping in new progressive candidates while letting less progressive Democrats be. This is actually how the New Deal coalition was built. The Southern conservatives remained in the fold--creating a natural sizable majority--but were progressively more marginalized within the Democratic caucus. In fact, it has been convincingly argued that FDR's hubris in first attempting to pack the Supreme Court in 1937 and then trying to "purge" the Congress of uncooperative Democrats in the 1938 primaries was the first steps in the GOP comeback. 1938 did end up being the first time the Republicans made gains since 1928.
Likewise, there are plenty of current Democratic officeholders who are "wrong" on various core issues. The recent eruption of bile here over the Iraq supplemental displayed how easy it is to feel betrayed by this and vow revenge. For the same reason that Judas is more reviled than Pilate, it is much harder to swallow--at a gut level--your supposed allies voting against your interests than it is to stomach your enemies doing what they do to wreak havoc with your interests. This is true regardless of who is most responsible for the bad outcome, which is usually a situation where the other side pushes something bad and some on our side let it happen.
But if we get beyond this gut-level frustration, there is no reason why we as progressives cannot build our movement without tearing down those who play the Judas in Washington. There is no reason we cannot promote ideology and promote progressive candidates for openings, either challenging Republicans or in open seat races, but "live and let live" with the Blue Dogs of the world. The only reason that they can throw monkey wrenches now is that the majority is small enough that they hold the balance of power. If it were larger, and if the caucus were dominated by progressives, you'd get progressive policies without having to expend time, money, and energy turning fire on members of our own party.
In the short term, I continue to believe that working to elect any old Democrat (in addition to aggressively promoting rising progressive stars) is in the best interest of any movement. In the long term, however, it is very easy to focus so much on this goal that you lose sight of the broader movement (as I will admit I am often guilty of). But as long as that is the goal, there is no reason that one cannot both work to get more and more progressives in government both at the federal and more localized levels and work to, say, reelect Sen. Ben Nelson in Nebraska.
Naysayers have said that supporting the Nelsons of the world stifles progressive candidates running very different campaigns from making inroads into the supposedly red areas where the Blue Dogs sit. The striking success of candidates like Jim Esch and Scott Kleeb last year should put that to rest. Indeed if anything is to give in this supposed "conflict" it will be that as the Kleebs and Eschs of the world bring Nebraskans around to more progressive thinking, Nelson will move in that direction as well. We must show the incumbents who got where they are through triangulation and centrism that there is another viable path available. You don't do that by putting a price on their head in the manner of the Club for Growth.
I titled this as a response to eugene, but I realize that he did not call for the wholesale attack on the non-progressives in the party and was mainly issuing a call to action for progressives to get out and work to reshape the party--which I wholeheartedly support. Still, there is a sense that a key task of the progressive movemnet is to electorally clear out "dead wood." I would say that it is almost always better in the long run to work elsewhere to diminish the dead wood's sway within the caucus.