I just updated this chart with the latest -- Clinton does have a resilient base, but Barack's growth has been steady, even as Clinton supposedly has hit her stride.
It's a reminder that her recent success is more of a function of when states chose to have their primaries than it is of any underlying trend. Indeed, if the nominating calendar had been flipped and the states that voted in March, April, and May had gone in January, I'd bet that Barack would not have done as well as he did.

If you think about what Barack Obama has pulled off this year, it's actually like defeating sitting or former vice president of your own party for the nomination -- and I can't remember the last time that happened.
Think about it: in 2000, Gore won the nomination; in 1988, Bush won; in 1984, Mondale won; in 1976, Ford won (though he was president at the time); in 1968, Nixon and Humphrey won; in 1960, Nixon won.
Pretty impressive.
© Jed Lewison