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No 3rd party can mathematically exist until fptp voting system is abolished and replaced with ranked choice or a similar alternative.
Simple as that.  Change FPTP first, then you can start talking about 3rd parties
I just want someone left wing >

Because of the dissatisfaction with the Dems and Reps, several news outlets have been claiming the USA needs a "third party" (usually with younger candidates).  However, we have more than three parties right now, and they offer younger candidates!  Here is a partial list (sorted alphabetically).  More info can be seen by Googling the name(s) below:  

American Independent  
Constitution Party  
Democratic  
Democratic Socialists  
Forward  
Green Party  
Independent  
Libertarian  
No Labels 
Peace and Freedom  
Reform  
Renew America
Republican  
Tea Party  

Each of those political parties claim to adhere to somewhat different philosophies.  The major difference, however, seems to be their funding.  The two major parties have significant funding.  The "wannabees" can't seem to get sufficient support.
So, the talk about "we need a third party" is really a complaint about the two major parties ... and those who espouse the "third party" seem to be unable to grasp the necessary fundamentals for significant recognition.  However, at least some of these third parties seem to be able to siphon votes from the major party that is most similar in goals, thus defeating their supposed purpose.
Botom line:
An increasing number of people want a third party, but have no idea or suggestions how to get a successful one.