Go form a message board called "Progressive Underground" if they cannot stay with the Democratic Party? Though this may sound like snark it is a serious point because of the UK experience.
Back in the bad old days of the 1980s and 1990s the UK was governed by a Conservative Party was able to move further and further to the right because of an ineffectual Labour Party that had surrendered to its extreme left wing at an organising level, and to inept or venal representatives on elected bodies. The reason for this ineffectiveness was that only the extremists were willing to do the hard work of organising because there were never any tangible results in the way of power, the inept representative were in place because they were harmless and the venal ones because they were corrupt and corrupting. Then along came John Smith, who began to change the old party and bring it back into the heart of politics.
John did this by embracing values nearer the centre and shutting down the noisy rhetoric from the far left wing, yet despite this John kept the left with the party and kept the party on the left. Because of John's sterling work Labour again became a competitive political party - but then he died and his place was taken by Anthony Charles Lynton Blair and the "New Labour" project. The Left resigned and moved into a multitude of minuscule campaigns. This moved the party into the centre right ground abdicated by the Conservative party whilst limiting the Labour social agenda.
This relates to the current problems of the Democratic Party because your "John Smith" has been elected President. Please do not think that Dr John Dean was your equivalent, for his British mirror is the much maligned and deeply underestimated Neil Kinnock; you even have a parallel Tony Benn in Dennis Kucinich.
OK, enough with the parallels - how does this relate to the current problems faced by people on this board? It relates because if the left separates itself from the main body of the Democratic Party it will allow centrists and deep right wingers more room to operate, again leaving the left isolated.
Why should the left separate? Is it because, for the first time in US history, there is a Healthcare bill which admits the principle that US citizens should have healthcare as a right? Remember however flawed this bill is it is this principle which is important not the flim-flammery that attracts the left wing ire.
Should the Left separate because the President has failed to initiate all of the policies they prefer instantly? Well, sorry, but in President Obama's view he cannot act in that dictatorial manner. Read "The Audacity of Hope" because in that book President Obama makes it clear that he is a constitutionalist who believes in the 3 arms of government. He may be head of state but his job is not making laws, for that is the role of Congress; nor should he judge the constitutionality of laws, for that is the ambit of the Courts. I believe from reading, that the President sees his job as the governance of the executive, the directing of policy and protecting the Constitution.
What role is left for the Left? Should they pick up their toys and leave, which will just allow right wing Democrats to pull the party to the right? Or should they stay and do what they do best, organising and working for more progressive Congressmen. The left must nurture the roots of the party, ensure that the RW are challenged at the Primary stage and, if necessary, removed. If the left accomplish this then the agenda of the left will, eventually, be fulfilled.
