[WS:] It may be an obstacle, but I do not think it is the primary one.
There is enough money in unions and liberal quarters of the society to fund a decent campaign, left or otherwise.
In my view, the primary obstacle is that the US society is by an large petit bourgeois. They love their country but hate paying taxes, they believe in god and hate godless communism/secular humanism, they think their little podunk is the center of the world and they are suspicious of outsiders, they think of themselves as self-made rugged individuals and their "wet dream"
is to own a small business and live in a cozy, familiar place, and be envied by their neighbors. Anything that left has to offer - from workers right to organize, to social justice and equality and to international solidarity has zero appeal to them.
I am pretty certain that even an European-style social democrat (not to mention a socialist) would not get more than a quarter of the popular vote here, and even that would be only under most favorable circumstances. This is why those are sympathetic to left causes would not give any money to a left candidate - it would be an utter waste of resources.
Wojtek