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%.
Half of all "Protestants" are not affiliated with a Church, nor do they attend Church. Therefore, they are not even Christian in name only.
Counting them as Protestants is the same as counting lapsed Catholics as Catholics.