Well, going by what
this page says...
In 2000 there were a little over 35 million Hispanic/Latino people in the US, which had a total population of 281 million. That's a little over 12% of the population. In 2007, this had become 45 million Hispanics/Latinos out of 301 million people. That's about 15% of the population.
According to
this document, about two-thirds of Latinos/Hispanics in the US are of Mexican origin.
So it seems that in 2001, 8% of the US population was Mexican in origin, and in 2007, this had risen to 10%. So it rose by two percentage points over six years, or very roughly, one percentage point every three years. It seems that in order to reach 50% of the US population, it would take another 120 years - assuming that things continue at the same rate, which is a very rash assumption. However, to get an
overwhelming majority of Catholics via this mechanism, I'd say you'd need at least 75% Mexican, which would take another seventy-five years. So we're talking about two centuries in total. And that assumes that the level of growth remains constant over that period, which I don't think is very likely. How can we possibly know what levels of immigration the United States will be experiencing a century from now, let alone two centuries?
(Note: all of this assumes that all Mexicans are Catholic, which in fact they are not - according to
this page only 75% of them are. But then, according to
this page, 24% of Americans are already Catholic, so perhaps these cancel each other out. I should add that, throughout Latin America, Protestant Pentecostalism is growing at an incredible pace, so I think it would be very rash to assume that a majority of these Mexican immigrants into the US in a century's time, or even fifty years, will necessarily be Catholic at all. That obviously makes the proposed scenario even less probable.)
So it seems to me that this is a very unlikely means for the United States to become majority Catholic, and the probability of its happening within a timescale over which we can predict social or religious trends with any reasonable degree of accuracy is negligible. I think that if the United States is to become majority Catholic it would be far, far more likely to happen through some kind of mass conversion of the people who are already there, not by mass immigration of new people. But such a mass conversion also seems very unlikely, given the deep cultural entrenchment of Protestantism in so much of American society.