He offers the best and most succinct take I've seen on just how brilliantly Hillary has played Obama:
The most remarkable political triumph of this campaign was the Clinton campaign effectively defining Barack Obama's "new politics" as "not attacking Hillary Clinton by name." Obama, of course, could have defined the new politics however he wanted, from a focus on transformative policy to a willingness to call out the DC establishment. Instead, he let the Clinton camp define his message in a way advantageous to them. And given that his campaign has not been particularly bold on policy, he's been left with neither the rhetorical room nor the substantive beliefs with which to tackle the frontrunner. It was real incompetence on the part of Obama's handlers, and it's called into question his skills and, yes, ruthlessness, as a campaigner.
It's a cliche that, in politics, you can't allow your opponent to define you. Which makes it all the more amazing that Obama has allowed Hillary to do just that.