In contrast to cooperating Republicans, the Democrats and the Left in the US are perpetually divided. The Democratic Party is divided into its progressive and "centrist" (Blue Dog) factions. In Congress, Democratic party discipline is weak. The Democratic leadership (including President Obama) is centrist and naively bipartisan. Moreover, even within the progressive wing there is considerable acrimony and division between the hard-core progressives and the soft-core progressives.
Soft-core progressives acknowledge the failings of corporate Dems and of the Obama Administration, but they believe that there are plenty of good Democrats, that it's useful to push the Democrats leftward, and that supporting third-party candidates will likely just empower the GOP, who are, after all, progressives' worst nightmare. Even the corporatist Dems aren't generally as evil as the Republicans. Progressives are outnumbered, outgunned, and outspent. Splitting off into a third-party would still leave them outnumbered, outgunned, and outspent. This argument is made more fully here.
On websites like OpEdNews, there are often pitched battles between hard-core progressives and soft-core progressives, especially in the Comments sections of articles.
This is a tough issue: when to submit a principled/protest vote. I think it depends on the details. For me, if there's a viable non-progressive candidate or if my vote won't help throw the election to a much worse Republican, I'll vote for the third-party person or the progressive primary challenger. Usually, though, there's no viable alternative and the Republican opponent is much worse than the (possibly corporate) Dem. The Republican Noise Machine of Fox News and AM talk radio has convinced tens of millions of middle class Americans that the Dems and Big Government are the problem.
Some critics of Obama and the Dems are, I suspect, Republican moles, aiming to incite division on the Left.
Some hard-core progressives are, no doubt, socialists or communists, and their opposition to the Democratic Party runs deep. They will never work within the Democratic Party. Let's call such hard-core progressives the "super-hard core." Given the realities of American politics, the hopes of the super-hard core for a socialist America seem quixotic. Besides, I do not support socialism or communism. I want a European style mixed economy, with a balance between private and public control.
But I know many hard-core progressives who are not socialists and who like and vote for many Democratic candidates. They're the "medium hard-core." Their unwillingness to work within the Democratic Party is a shame, because there seems to be little ideological difference between such medium hard-core progressives and the soft-core progressives. Both factions do not want to eliminate private property or to nationalize most industry. But the medium-hard core progressives reject the Democratic Party and hold out hope for a viable third party, even though it may be decades before a viable third party arises that can challenge the Dems and the GOP.
I see this effect particularly in local politics: I know of several progressive, affluent anti-war friends who refuse to participate in the local Democratic Party. Much of the reason is, no doubt, ideological. But part, I bet, is the unpleasantness of attending meetings and of having to defer to the people who often run such meetings. Local politics is boring and tedious and often puerile.
I think some of the hard-core progressives are simply inveterate iconoclasts, independents, and dissidents. There's freedom and a sense of superiority when you're in opposition, but it's not always a productive place to be. The internal politics of third parties is pretty damn ugly too. You gotta work with what you got. What we got is a mixed Democratic Party that produces some good candidates and some decent bills.
The point of this article is this: the animosity between soft-core and medium hard-core progressives is kinda silly. Both factions share the same goals and values (economic justice, civil rights, women's rights, gay rights, environmentalism, unionism, an end to militarism, and justice & accountability). They just disagree on techniques.
I still hold hope for the Democratic Party, but I respect those who choose not to. Yet the status quo, with the Left divided, is enervating. Either all (soft and medium hard-core) progressives should bolt and form a new party, or all should work within the Democratic Party to reform it. I prefer the latter path, because I think that it's easier to influence the Democratic Party as its progressive wing than it is to influence the entire nation as an out-of-power, outgunned, outnumbered, and outspent third party. If America had a parliamentary political system, then third-parties could wield tremendous influence. But in our political system, third parties can usually end up aiding the enemy.