I don't buy into the argument that because they attack her, they must be "scared" of her. Maybe they just detest her policies and image because it's the opposite of their own. But that doesn't equate to fear that she's a great candidate.
For as long as I can remember, Ted Kennedy was the poster child for liberalism in this country, and was rightly despised for it by many Americans, especially conservatives. But just because we hated what he stood for and went after him whenever possible didn't mean that we feared he would be a great Presidential candidate. As I recall, we'd have loved for him to have been the Democratic nominee because we were convinced he was too liberal and would get stomped.
Still hated him (in the political sense), though.
But they don't attack her policies. They primarily attack her personally. When they attack her policies, they attack conservatism, and that usually doesn't work too well.
Actually, IMHO, neither tactic will work against Sarah in the long run...