Basically, it's this: John McCain only endangers Democratic chances of victory this November, but Sarah Palin is an existential threat to the Democratic Party.
She threatens a core element of the party's base - women.
Democrats can't stomach seeing the feminist movement's impetus for greater female political participation and empowerment "hijacked" by a pro-life woman who espouses traditional values. They must obliterate her, lest her popularity eat away at their party's core.
So the Democrats are hysterical in their attacks on her. South Carolina's Democratic Party chairwoman, Carol Fowler (wife of a national party chairman), said that the only qualification Palin had for vice president was that she hadn't had an abortion. Tabloids are digging up dirt on Palin's children. And liberal bloggers have suggested that Palin would neglect her children if she were elected (while the Democratic candidate has young children at home, too).
That liberals would resort to such blatant sexism shows their desperation.