"The framework of 1972 has given radicals ever since a stake in the Democratic Party. It's also been the source of a lot of frustration. The role of the left is mostly invisible and thus different than 1930s and 1940s when clearly identified radical caucuses were common. For one thing, it is hard to know what 'the left' is and who belongs to it. The contemporary left has no structure nor even a definite label. 'Progressive' has become a vague identifier, but the term is used so loosely as to be almost meaningless.
"Secondly, the left has been largely shut out of national level Democratic Party campaigns since 1972. Only once has there been anything like the Sanders campaign. In 1984, Jessie Jackson’s Rainbow Coalition primary challenge turned into a grand crusade that energized and expanded the left in a manner not unlike 2016. Otherwise centrists have commanded the party's main stage."
James N. Gregory at The Conversation quickly traces the story of left-wing Democrats over the past eighty years.