Wow, I never heard that. Can you post a link or let me know where you learned this?
Just add up the numbers in any religious census of the US (i.e., look in the World Almanac or the Statistical Abstract published the by the Census, or google "American Religious affiliation" or similar). There are about 68 million Catholics, and about 70 million affiliated Protestants and others of all stripes. The number of Catholics does not include at least 10-20 million more Catholics who have left off the normal practice of the faith and affiliation with a parish, but not taken up another formal affiliation.
Roughly: 8 million Lutherans, 2 million Episcopalians, 12 million Methodists, 35 million Baptists, 1 million Adventists, 4 million Presbyterians, 10 million Pentecostals, a few million assorted others.
All Christians 84% (all includes Mormon, Orthodox, etc.)
Protestant 52%
Catholic 24%
So Catholics are 24% of all Americans, or 28% (24/84) of all Christians.
Similarly, according to the CIA World Factbook:
Protestant 52%, Roman Catholic 24%, Mormon 2%, Jewish 1%, Muslim 1%, other 10%, none 10% (2002 est.)
According to these 2002 statistics, Christians are 78% of the U.S. population (Catholic + Protestant + Mormon), and Catholics are 24% of the U.S. population, or 31% of the total U.S. Christian population.
So in the U.S., Catholics are 28 - 31% of all Christians, not 50%.