First, let’s look at voter turnout. It dropped to 129.2 million in 2012 from 131.5 million in 2008, according to David Leip’s Atlas of U.S. Elections.

But the drop-off was not among conservatives. According to exit polls, self-identified conservatives made up 35% of the 2012 turnout, and 82% of them voted for Mr. Romney. This translates into about 45.2 million conservatives who turned out—roughly 531,000 more than in 2008.

In 2008 conservatives were 34% of the turnout, and 78% voted for John McCain. So Mr. Romney got around 2.2 million more conservative voters than Mr. McCain—and the conservative share of the 2012 electorate was the highest since exit polls began asking voters about their political leanings in 1976.