Jim Lindgren at the Volokh Conspiracy is puzzled that the Obama campaign has been aggressively going after Stanley Kurtz, a right-wing hack who is trying to smear Obama by linking him to ex-60s radical bomber Bill Ayers, without making more vigorous efforts to denounce Ayers himself.  This comes perilously close to concern trolling, but I'll respond briefly:

1) The substance of Obama's response to these accusations is that he doesn't know Ayers all that well.  There's no particular reason Obama should be denouncing people he doesn't have any close connection with.

2) Accordingly, Lindgren's suggestion that this smear would go away if Obama were to denounce Ayers doesn't work:  the immediate response would be, if you're denouncing him, he must have been your close friend, so why didn't you do that before?

3) The fact that the Obama campaign is going after Kurtz, who is aggressively trying to smear Obama, is not surprising.  Lindgren's suggestion that the campaign should let the smear spread without comment is dubious:  millions of dollars are being spent, right now, to spread this smear. 

By way of comparison:  suppose I claimed that I had evidence that John McCain is under the influence of North Vietnamese mind control techniques, and tried to get booked on the Today show to discuss that.  According to Lindgren, the best course of action for the McCain campaign would be to denounce the North Vietnamese and say nothing at all about me.