"Progressives should not make the same mistake that Ernst Thälmann made in 1932. The leader of the German Communist Party, Thälmann saw mainstream liberals as his enemies, and so the center and left never joined forces against the Nazis. Thälmann famously said that ‘some Nazi trees must not be allowed to overshadow a forest’ of social democrats, whom he sneeringly called ‘social fascists.’
After Adolf Hitler gained power in 1933, Thälmann was arrested. He was shot on Hitler’s orders in Buchenwald concentration camp in 1944."
The only way a third party would be viable in the US is if it grew organically from small, local races that aren’t captured by large donors. A dedicated group of volunteers knocking on doors and spreading a message can have a real effect in those races. Get a few candidates in office and start doing some good, and a party can grow around it. Draw up a blueprint for how you did it, and spread it around to other towns and cities, making allies with other local groups as they spring up.
Is that easy to do? Of course not, but that would be a viable path for the formation of a functioning third party.
The moment it makes waves on even a local level, one or both major parties would begin to invest resources in crushing it wherever it appeared.