Jay Cost:

It is hard to identify a momentum effect here. Clinton's losses in the contests are as explicable as any of her losses before or on Super Tuesday. Obama has systematically won states that play to his particular strengths since the Iowa caucus. So has Clinton. Her problem has been that she has not had any good states in the last week.

And yet ... campaigns are dynamic things, and momentum matters. Obama has the time to make the sell - something that he lacked in the Super Tuesday states.

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