Since the "kitchen sink" strategy was deployed, Clinton and her surrogates have been throwing every conceivable attack, smear, and insult at Obama. Having accepted internally that they can't win the primary by any objective measure, their only hope is to so damage Obama that he becomes nonviable for the general election and force the convention to nominate her. While these are surely desperate and despicable tactics, they appear to be working.
Clinton has adroitly exploited the lingering sympathy for her as the "real victim" of Monicagate by repeatedly playing the victim in this campaign. First it was the "mean boys beating up on the poor girl" meme, which failed to fly. Then it was the "no one gives poor little ol' me a chance in any primary", which helped to some degree in the New Hampshire win. The unfunny SNL skits on how the press was in the tank for Obama followed, and she made hay off those. Big, mean Samantha Power then called Clinton a "monster" (which strikes me personally as a rather mild insult given the gratuitous campaign so far) and Clinton screamed about how out of bounds and beyond the pale it was that Power was speedily dismissed.
Now the Ferraro flap -- a high-ranking and admittedly historic Clinton campaigner makes plainly wrong and racist comments about Obama and then refuses to retract them in the obligatory "did-you-really-mean-that" followup interview. This seems an improvement on the victim strategy; instead of people being mean to poor little Hillary, she's tying her feelings of undelivered entitlement to those of any working-class white person who feels shafted by an affirmative action plan. This is sick, and it's going to work. How might Obama effectively answer this disgusting display?
I think you can go on the offensive without being negative. Attacking her directly in the same way she's been attacking him will do more to feed the victimization narrative, and of course it doesn't agree well with the "new politics" brand (which I believe he means honestly and which I also believe has real appeal to voters). Clinton's supposed strong points are government experience and foreign policy expertise. In the best Rovian tradition, these are not her strengths but her weaknesses, and she should be attacked by Obama surrogates.
I also think that the more Clinton plays this card -- and I doubt she's savvy enough to know when she's about to overplay it -- the more it will reinforce the existing image of her as a shrill, emasculating bitch. This will become less about white vs. black and more about woman vs. man. If the former frame is used, she has the advantage; if the latter, he has it.
Finally, there's one simple thing Obama can do that will show toughness, dedication to what is right, and a willingness to fight. If Clinton, as she seems, tacitly approves of Ferraro's tripe, then he should rehire Power. Getting rid of her was a peace offering; agreeing to put some things beyond the pale of acceptable campaigning, including the "monster" remark. Yet Ferraro's crap is much worse than "monster", and if it benefits Clinton among some key demographic, she's happy to allow it to continue and only issue mild McCain-Hageeish tsk-tsks. Well, if that's how she wants to play it, then bringing Power back shows that Obama tried to take the high road, Clinton spit on him, and now he's doing what he thinks is best regardless of what she screams. By showing his dedication to what he thinks is right, he makes Clinton look like the opportunistic elitist she is and retakes control of the rules of this stupid game.