The Pew Forum on Religion & Public Life has released their U.S. Religious Landscape Survey as well as a number of articles analyzing the survey.
One such article ( Study Finds Americans Fluid in Their Religious Affiliation ) gives the breakdown of Christians in the USA as -Evangelical Protestant: 26 percent -Mainline Protestant: 22 percent -Catholic: 24 percent.....pointing out that Evangelical Protestants outnumber Catholics in America.
This is a bit misleading as the Catholic Church is the largest denomination in the country.
If you go to the survey and examine the breakdown, you'll find that the Evangelical Protestants get that 26% number because that group comprises more than 50 different denominations. The largest denomination in that group is the Southern Baptist Convention which makes up less than 7% of the population.
The, so called, Mainline Protestant group (22%) contain nearly 30 denominations; the largest being United Methodists at less than 6%.
When you look at the actual breakdown of the denominations by percentages, the fact that Catholics make up one in four American is impressive indeed.