“The most volatile factor in our politics in the first decade of the 21st century,” Michael Barone writes in the 2010 Almanac of American Politics, “is the balance of enthusiasm.” Enthusiastic Republicans turned out in droves to support the GOP and President Bush in 2002 and 2004. By 2006 and 2008, however, Bush’s popularity was tanking and the Democrats were bent on “taking the country back” from the right-wing attack/noise/nihilist machine. It’s common-sense: The party with the most enthusiastic supporters wins. But it’s refreshing to see a truism unfold before your eyes.

Here’s the nonpartisan election analyst Amy Walter, for example: