Via Gallup a major reason why the U.S. can sustain its power in the coming decades:
Gallup estimates 6.2 million Mexican adults say they would like to move permanently to the United States if given the chance. That's close to half of the 14 million Mexicans -- or 19% of the adult population -- who say they would like to resettle somewhere else; would-be migrants in Mexico choose Canada and Spain as their other top desired destinations...
Keeping in mind that Gallup's numbers reflect desire rather than actual migration rates, Mexico's roughly 6.2 million would-be migrants to the U.S. pale in comparison with the estimated 22.9 million adults who would come from China, 17.1 million from India, and 16.6 million from Nigeria. Ethiopia, Bangladesh, and Brazil would also send more migrants than Mexico.
As Gallup goes on to note, if everyone who said they wanted to move to the United States actually did so, the population of the U.S. would grow by 60 percent. The ability to attract immigrants is not all good, of course, but it does speak to the country's capacity to regenerate itself and stave off a decline in population. America's two major great power rivals - China and Russia - can boast of no such attraction.