Matt Bai nails it:
It must be a kind of nightmare for both Clintons to be running, at this moment, against a talented black man, to be caught in an existential choice between losing their mythical status in the black community or possibly losing to a candidate they feel certain does not deserve to win. But only they can afford to be concerned right now with their own historical legacy, about seeing all that they have accomplished on behalf of their party and its commitment to fairness and equality blown away in the space of a few months. No one else is going to protect all that for them. No one around them is going to take the long view, because that’s not the way supporters think.
No one expects Mrs. Clinton to stand down and let Mr. Obama make his case unchallenged. She could, however, send a clear message to the cogs in the machinery she’s built that there is a line she will not cross. She could tell her Nevada allies that the job of the Democratic Party she grew up in is to make it easier for people to caucus, not harder. She could tell Robert Johnson that he needs to apologize, the same way she forced Bill Shaheen, her New Hampshire co-chairman, to resign last month. She can make it plain to all those people trying to get jobs in the next Clinton Administration that there is way to win—a rough and combative way, even—that nonetheless won’t destroy all the good that the Clintons, at least for a lot of Democrats, have come to represent.
Sadly, the evidence seems to be accumulating that this is not the plan.