Nate Silver put out a great article and graphic on what Bernie Sanders needs in each state to secure nomination. The right hand column is what he needs, and the left is what is (or was) expected based on Clinton’s 12 point national lead.
As you can see, while beating expectations, he fell short in both Iowa, where he lost by less than a percentage point, and in New Hampshire where he won by over 20 points. He needed to win by 6 and 29 points respectively to keep to the path outlined. He will need to make up ground in later states.
Not impossible, but not easy by any means.
Also Mr. Silver includes the following caveat:
Don’t get too attached to these: The state-by-state estimates are pretty rough. But they’re calibrated in such a way2 so as to provide a reasonable benchmark of what a 50-50 race would look like. Maybe Michigan is less favorable to Sanders than this estimate holds, for example. That’s fine, but it means he’ll need to make up ground in another state.