The vice presidential debate was uniquely important because if Paul Ryan won it or did well, the Romney-Ryan ticket's momentum would be continued or speed up. If he did not, that momentum would slow or stop. So the night carried implications.

The debate, obviously, was the Republican versus the Democrat, a particular kind of conservative versus a particular kind of liberal, one philosophical approach versus another. Beyond that there was some iconic weight to it. It was age versus youth; full, white-haired man versus lean,...