Super delegates.
As the Times editorial has it, the weight of party bigwigs can (and probably will) simply hand the nomination to Hillary Clinton. They might lose that ability to corrupt the "popular vote" if only Democratic voters were to vote. But we don't. We accept losing the power of our vote and then sit back and blame the political machine. Aside from anything else, we should feel embarrassed. Look at what the DNC did to New Hampshire:
In the New Hampshire primary last week, which Mrs. Clinton lost by 22 percentage points, Mr. Sanders won 15 of the state’s 24 pledged delegates, and Mrs. Clinton won nine. But because she has the support of six of the state’s eight unpledged superdelegates, including Senator Jeanne Shaheen and Gov. Maggie Hassan, she is virtually tied with Mr. Sanders in the New Hampshire delegate count. ...NYT
MoveOn is mobilizing an effort to get super delegates to back off. Maybe they'll succeed. But it doesn't solve anything until the DNC changes its ways.