Bill Clinton Speech Asks Undecided Voters To Hang On, Give Obama Another Chance

There's one vital reason that the head of a political party doesn't endorse anyone not in their own party, and that's about negatively influencing the turnout for other races in the state.  If you can't even muster up the enthusiasm for your own party's nominee, your party's voters won't.  They may not turn out to vote at all, which means the party loses all the way down the ticket.  That's one reason why Republicans have taken over so many state houses.
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