Until recent weeks, one of the least understood aspects of the Democrats' primary contest was the role of superdelegates. These are Democratic Party insiders, members of Congress, and other officials who can cast ballots... But now these unelected delegates are coming in for a close inspection, because neither Hillary Clinton nor Barack Obama can win their party's nomination without superdelegate support. The big Pennsylvania primary on April 22, for example, has only 158 delegates at stake (each of them will be pledged to support one of the candidates). By comparison, there are a total of 795 superdelegates, none of whom...