Like that proverbial donkey stranded between those two bales of hay, the American left has looked longingly in the direction both of third parties and the Democratic Party -- and gone nowhere. Considering all of this history, along with the experience of the American labor movement and the European Left, the Primary Route argues that the route to relevance for an American electoral left has long been hiding in plain sight. It runs through the presidential primaries. And until the left figures out how to put its ideas out there in the main event every four years, mainstream relevance will continue to elude it.