He's just had a bad loss in Pennsylvania (though it was expected, the cash he piled in should have resulted in a better outcome), he has accused traditional working class voters of clinging to religion, guns, and harbouring hostility towards immigrants through bitterness, and the mad pastor is on the prowl. Obama's poll position is remarkably solid.
David Brooks in the New York Times has part of the answer. America is divided, this is a demographic election. Obama and Clinton have different appeals to different demographics and that is just playing out. As E.J.Dionne of the Washington Post has said, this primary is just like the census. You know how each candidate is going to do in each group then you can predict the outcome in any state once you know its demographic configuration.
What I have seen less of and will look out for is why Obama and Clinton have different appeals to different demographics. There will be positive and negative reasons. A major reason for Obama's difficulty in most states with the white, working class could well be that the familiar has more appeal when you find yourself in an economically and socially precarious position. So it may not just be guns and god that you cling to. Perhaps you cling to Clinton as well?
Postscript: The mad pastor problem is becoming corrosive for Obama and it is clear that his brilliant speech on race has failed to limit the damage. I wonder what Jeremiah Wright's tour of Town Halls and TV studios is designed to achieve. Self-rehabilitation would seem to be the obvious motivation but the more he speaks, the more ridiculous he sounds. Perhaps he is not throwing Obama under a bus but, rather, encouraging Obama to throw him under a bus. Obama has a difficult choice to make. Should he disown his former pastor and former church? It may become inevitable. If it does, it is a decision he needs to make sooner rather than later.