“April is the cruelest month,” wrote T.S. Eliot, and it’s difficult to imagine that Barack Obama and his people would in any way disagree. For the past 29 days, Obama has suffered through the rockiest stretch of his bid for the Democratic nomination — a period in which he rarely won a news cycle, as a series of blunders and controversies made it seem increasingly as though he was losing control of the narrative of his campaign and of his public image. Then came his extraordinary press conference yesterday, in which Obama sought both to make a clean break with...