Julian Zelizer: Why Democrats Need Labor Unions

Julian Zelizer is a professor of history and public affairs at Princeton University. He is the author of "Jimmy Carter" and of the new book "Governing America."

Princeton, New Jersey (CNN) -- Since the Democrats chose to host their convention in Charlotte, North Carolina, in a state that has not been hospitable to unions, organized labor is going to have a rally to focus attention on its key issues in Philadelphia on August 11.

The rally is meant to send a message to the Democratic leadership, as well as to Republicans, that many workers feel as if they don't have a voice in the two-party system. As AFL-CIO President Rich Trumka said to members, the rally will "give us an opportunity to connect the faces of ordinary Americans to the basic issues affecting working people in our country while providing an important liftoff to our Labor 2012 political program for the fall."

The rally is indicative of a larger tension that has plagued the Democratic coalition for several decades. Since the 1960s, organized labor, once the pillar of the Democratic Party, has often been taken for granted or even treated with hostility....