Where did you get that stat? I have read that according to Pew Research Center, Evangelical Christians make up p 26 percent of adults in the country, and about 26 percent of eligible voters, and 79 percent of evangelicals voted for Mitt Romney in 2012, a wider margin than they did for Sen. John McCain four years ago, while Obama won the 48 percent of the electorate that was Christian and not Protestant or Mormon -- 50%-48% among Catholics (25% of the electorate) and 50%-49% of "Other Christians" (23% of the electorate).
But the U.S. Census says there are more than 311 million people in the United States. If evangelical adults are 26 percent of them, then there would be 80 million potential voters. Evangelicals made up 26 percent of the 123 million votes that had been counted (about the time the article was written) in the 2008 election, therefore, about 32 million evangelicals voted – 40% of their population.
But there’s a need for context here: (1) They make up just 14 percent of the registered-voter base in the NBC/Wall Street Journal poll. They outpaced that percentage in the presidential election, (2) This is true every other demographic group in the country as well. - http://firstread.nbcnews.com/_news/2012/11/16/15219396-no-its-not-christians-fault-obama-won?lite
More stats and even more by God's grace.
And here is a detailed chart from Pew:
http://www.pewresearch.org/fact-tank/2016/02/23/u-s-religious-groups-and-their-political-leanings/