Labor unions have historically been a key factor in determining the winner of the Democratic primary, with the endorsement of the AFL-CIO in particular pushing the candidacies of Walter Mondale in 1984 and Al Gore in 2000 over the edge. This year, however, unions are split in their candidate affiliations, paving the way for other constituencies such as minorities, women, and liberal activists to play a larger role than ever before. Political experts point to the rare occurrence of three legitimate candidates early in the primary season as cause for unions to hedge their bets and wait for a clear frontrunner to endorse.
See "Once-monolithic labor vote splits on 2008 campaign", David Broder, San Jose Mercury News, February 19, 2008