They love to badmouth Sarah because she is usually right and is a threat to 'them'.
It is a play right out of the play book of every modern liberals' hero, Saul Alinsky ('Rules for Radicals').
RULE 12: Pick the target, freeze it, personalize it, and polarize it.. Cut off the support network and isolate the target from sympathy. Go after people and not institutions; people hurt faster than institutions. (This is cruel, but very effective. Direct, personalized criticism and ridicule works.)
Seriously, look this guy up. Everything Obama or any other hard left liberal does, is right out of this guy's play book. They idolize him. Very scary.