Take whatever you thought Clinton’s chances of winning the nomination before [John] Lewis’ decision and divide that number by as much as two — those are the odds of her winning now.
I suppose if people like Halperin treat Lewis's reversal that way this could become a self-fulfilling prophecy. (Halperin notes that "those in the party and the press who want to write off her chances will be able to make a big deal about this development," even as his home page makes a big deal out of the announcement, guaranteeing everyone in the party and the press will make a big deal out of the announcement.)
But it strikes me as a big overstatement. For reasons Noam spelled out, black leaders like Lewis face obvious and intense pressure to get behind Obama in ways that most superdelegates don't.
Moreover, incidentally, this isn't so much Lewis flipping as Lewis flipping back. See this, from January 2007:
Atlanta Congressman John Lewis, who was a key leader in the civil rights efforts of the 1960s, said he was an early supporter of Bill Clinton and admired his wife, but called Obama "refreshing" and said he would back him.
Apparently subsequent cajoling from Bill Clinton reeled Lewis back into the fold.
Obviously this is a nasty development for Hillary. Among other things, it's one more reminder of Bill's reduced stature with his former African-American allies. It hurts. But an event that cuts her odds in half? That's how I would describe a big loss in Wisconsin. Not this.