I don't think US liberals have any specific connection to the Democratic party apparatus. The party does whatever regardless of their base's demands. The Democratic Party has successfully become a rorschach for liberals, they see it doing whatever they want it to be doing.

Liberals within the party or large donors are the actual apparatus and they're all lanyard wearing ghouls. I can't think of any instance since the civil rights movement or Vietnam protests that widespread public sentiment made the Democrats do anything.