"If I'm reading the Pew article correctly...Nondenominational Protestants...Pentecostals... are underrepresented."

Not really, MattinTX. Non-denominational Protestants and Pentecostals are always put under the generic label "Protestant," in Congress. In Pew, Gallup and other pollsters, the "Protestant" label includes Evangelical, mainline, Pentecostal, and "non-denomination" along with any and all other non-Catholic, non-Mormon, and non-Orthodox Christian groups in between them and among them. Thus when Pew researchers say 46% of the US population identify as Protestant, for example, they put all of the above self-identified, highly diversified groups (in Congress, too) -- Evangelical, mainline, primitive Baptist, Southern Baptist, Black Baptist, Pentecostal, neo-Pentecostal, non-denominational Christian, etc. and hundreds more -- under the label "Protestant." Catholics, Mormons, Jews, NONES (more than 24%), Buddhists, Orthodox, Muslims, etc., make up the other 56% of the population