According to the 2010 U.S. Religious Census of Religious Congregations & Memberships Study by the Association of Statisticians of American Religious Bodies, less than 50 percent of the people living in the United States identified themselves as Christian adherents in 2010. There were more than 150.6 million out of 310 million. Even so, candidates for the U.S. presidency still felt the need to vocalize the fact that they are Christian (while the opponent doesn't quite measure up in that respect). President Obama made a point during his first two years in office to stress his Christianity as if it were the membership card to the Oval Office. It would seem that the litmus test was already antiquated and thus needlessly constrictive on potential candidates.
The full essay is at "Christianity by State."