The essence and very beauty of the 'United States of America' is the intended expectation and acceptance of our diversity to include the acceptance of our religious diversity. Regardless of majority or minority - all are created equal...
Statistically speaking, according to the 'Religious Landscape Study' conducted by the Pew Research Center approximately 70% of US demographic are Christians, of which Catholics only make up 20% of the US landscape...
Relative to South America the demographic is seemingly substantially flipped: where an estimated 90% of South Americans are Christians with Catholics making up a significant 82% of their religious landscape.
IMHO - I do not foresee the US demographics ever shifting to the degree such that the hypothesis "if ever US is a Catholic majority nation" ever becomes a reality...