The Times useful piece today on McCain and evangelicals includes this familiar tidbit:
The instrumental role of evangelicals in Mr. Bush’s victory in 2004 over Senator John Kerry is an oft-repeated tale at this point. Mr. Bush’s openness about his personal faith and stances on social issues earned him a following among evangelicals, who represented about a quarter of the electorate in 2004. Exit polls in the 2004 election found that 78 percent of white “born again” or evangelical Protestants had voted for Mr. Bush.
It sounds like a daunting advantage. But, of course, the relevant comparison isn't between white evangelicals and voters generally. It's between white evangelicals and socially conservative (though not necessarily politically conservative), white working- and middle-class voters--which is to say, between white evangelicals and demographically similar non-evangelicals.
My sense is that Bush did enjoy an advantage over Kerry among evangelicals per se, but it wasn't so large. For example, I'd bet Bush got at least 65 percent of the group that was similar to white evangelicals in every way other than being evangelical.* Which isn't that much less than the 78 percent support he won from evangelicals.
The reason I point this out is that if Obama can just bring the GOP's evangelical margin down to 65-35 from 80-20, he'll have essentially neutralized the "evangelical" part of the evangelical vote.
*Kerry lost white working-class voters by 23 points. I'm sure the margin was significantly higher for socially-conservative working-class whites.