Kucinich
Because the policies of Biden, Richardson and Dodd do not diverge significantly from those of their higher-profile peers, I'll jump ahead to Kucinich, who I believe is responsible for pushing Edwards into what was previously an upper-tier vacuum on the left. Had Edwards failed to make this move, it would have invited the rise of an alternative candidate from below, all while exposing him to continued losses to Hillary on terrain that she dominates. Richardson is apparently the establishment preference in this regard, with his party connections and energy sector ties. But, like so many candidates, Richardson cannot meaningfully distinguish himself on policy grounds; thus, he is forced to wait in the wings for the colossal misstep of his first-tier opponents which might lend relevancy to his campaign. In contrast, Kucinich's legitimacy is drawn from popular social movements -- the anti-war, environmental, civil rights, and social justice causes, for example -- which doubtless alienate significant portions of the owning classes, thereby precluding the kind of financing which becoming president in this country necessarily requires, but nonetheless resonate deeply with large swaths of the public.