The big demographic story out of the 2012 presidential election may have been President Obama's domination of the Hispanic vote, and rightfully so. 

But as we close the book on the election, it bears noting that another less obvious bloc of key swing state voters helped the president win a second term.

They're the "nones" — that's the Pew Research Center's shorthand for the growing number of American voters who don't have a specific religious affiliation. Some are agnostic, some atheist, but more than half define themselves as either "religious" or "spiritual but not religious," Pew found in a recent survey.

They are typically younger, more socially liberal than their forebears, vote Democratic, and now make up nearly 20 percent of the country's population. Exit polls suggest that 12 percent of voters on Election Day were counted as "religiously unaffiliated."

"This really is a striking development in American politics," says Gregory Smith of the Pew Forum on Religion & Public Life. "There's no question that the religiously unaffiliated are a very important, politically consequential group."

The religiously unaffiliated voters are almost as strongly Democratic as white evangelicals are Republican, polls show.

Their overwhelming support of Obama proved crucial in a number of swing states where the president lost both the Catholic and Protestant vote by single and low-double digits, but won the "nones" by capturing 70-plus percent of their votes.