UNPLEDGED AT-LARGE delegates are "wild card" votes from a state. These make up about 20% of a state's delegates, and they don't have to declare any allegiances before attending the convention so no one knows ahead of time who they will vote for.
Unpledged delegates are usually members of the Democratic National Committee or another kind of elected party official like a representative or governor.
The only way a candidate can be assured a victory before the convention is if he wins enough pledged delegates to offset the number of unpledged delegates who could vote for someone else. On Mar. 11, John Kerry had amassed the 2,162 pledged delegates he needed to secure the party's nomination.17