I know there is a lot of talk about how hard it is to start a third political party and there is no doubt that the talk is true. Large political swings that realign the political parties in the US are rare, but the political history of the US is about the swings. Whigs and Tories, Bull Moose and Know Nothing parties. These things come and go and right now we have the appearance of a two party system: the dems on the left and the repubs on the right, but the truth is that we have dems in center/right and the repubs in right to hard right.
There is no significant left party in the US, only the 25 to 30% of us who identify as left/progressives/liberals/social democrats etc. and we are left to rail at the dem party to move left and lead the country with good public policy that works for all of us. The dem party leaders provide lip service, then do the bidding of the large political contributors - the deciders, the haves and have-mores who control the political agenda of the dem and repub party. Don't kid yourself about that. Just look at the need to provide Medicare for Everyone, a national health insurance policy that could/would have left the insurance industry scrambling to compete for Medigap insurance coverage, but instead we could not even get a public option, we got Health Insurance for Everyone - The Pay Up health care system doubled down on us.
So when will a political realignment occur that will put a left political party on the scene? Who knows? These changes are like the Arab Spring, the Occupy movement and more. The political shift will occur when the time is right. There is work to be done having the options available when the body politic wakes up and says, that's it, I have had it with the Dems and Repubs. Here are some options that are already established and waiting, or developing:
The Green Party - Another U.S. is Possible, Another Party is Necessary (a political party that is established and global)
Americans Elect - Pick a President, Not a Party (doesn't look like a political party in the making?)
Justice Party - Economic, Environmental, Social and Civic Justice for All (Rocky Anderson's party platform is the basis at this point)
Did I miss any?