It's the birthday of Dr. Martin Luther King, Jr., and his legacy is a guaranteed topic at tonight's Democratic debate in Las Vegas. For the past few days the Obama and Clinton campaigns -- and the candidates themselves -- have been exchanging barbs about civil rights and diversity. We hear what's been said and why it's touched such sensitive nerves. Does Bill Clinton deserve to be called America's "first black president?" Has Hillary inherited that legacy? Does Obama -- with a real chance of winning -- threaten an older generation of black civil rights leaders?