Kinda like "freedom's just another word for nothing left to lose". Hillary has nothing left to lose: her credibility (never her strong suit) is shot, her negatives are as high as can be, and anyone who isn't yet turned off by her is probably immune to any more antics.
So Hillary has the "freedom" to use whatever campaign tactics she wants, it can't hurt her anymore. She can distort the truth, or even blatantly lie, and it won't hurt her. She can sling mud all she wants, it won't hurt her. She can change her policy positions and make inconsistent statements, it won't hurt her. She can come across as the biggest wierdo ever for a mainstream presidential candidate, and it still won't hurt her. Whatever she can do to bring down Barack, or try to pull the wool over voters eyes, it may hurt him, but not her.
But Barack can't use these devious methods or his premise is tarnished - it would hurt his standing.
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That's her asymetric advantage - like Dick Cheney's terrorists who have no care for human life including their own, taking on a civilized nation that (in theory) cares about these things.
Barack, this will be good training for when you have to tackle the terrorist threat as President. Let's see you plan a winnable strategy against Hillary's asymetric advantage as she goes down fighting these next few weeks. Yes you can!