A ttending two political conventions back to back is like visiting two parallel universes: one conservative, one liberal; one overwhelmingly white, the other emphatically multicultural; and each one strangely confident that its candidate is on a steady course to victory.

In Tampa, Fla., Republican strategists insisted their faith was grounded in arithmetical determinism: the math of the sluggish economy. No president has ever been re-elected when unemployment was stuck as high as
8 percent, they pointed out.