According to the 2014 exit polls, 77% of the electorate was Christian. That’s almost identical to the percentage of Protestants in the general population (78%) according to the 2008 Pew U.S. Religious Landscape report. In both the exit polls and the Pew report 24% were Catholic and 2% were Mormon. Pew has Protestants at 51%, and Jehovahs Witnesses, Orthodox and other Christian denominations at less than 2% of the population. However, only 29% of exit poll respondents self-identified as Protestant, while a whopping 22% said some form of Christian other than Protestant, Catholic or Mormon.
I’m going to guess that Jehovah’s Witnesses, Quakers and Mennonites, and Russians, Greeks, Ukrainians, Copts, Keralites and Eritreans and people who’ve converted to those groups’ dominant faiths did not make up 22% of the electorate. Rather, it looks like nearly half of the Protestants who voted in 2014 do not identify as Protestant, or do not realize they are affiliated with a church or denomination that is Protestant.