Jay Cost has written that, in 2008, Barack Obama ran “a bandwagon campaign with a simple purpose. When your candidate lacks the experience traditionally thought to be necessary to run the government, and you have two wars and an economic slowdown, you need something to cover the gap. And that something was the impression that there was a broad, mass movement behind Obama.”

To some extent, the Obama campaign was clearly quite successful in creating that impression. Even now, most people don’t realize that Obama beat John McCain by 3.3 points less than the average margin by which Democrats won in House races that year. (Democrats won by 10.6 points in the House; Obama won by 7.3 points over McCain.)