This fact has been cynically exploited by pro-immigration lobbyists in the past. If America remains a country with moderate population density and low net population growth, it will be able to essentially ignore nasty population-control measures. The problem is not just the raw numbers of immigrants coming in, but the fact that they will then themselves reproduce, and at a higher rate than native Americans. This is particularly so the more they come from high-fertility Third World societies. Native Americans, like other citizens of highly developed societies like Western Europe and Japan, have a fertility rate that is at or slightly below replacement level, which is what a mature society that has found its natural level needs and can sustain, particularly with advancing consumer lifestyles.
Immigrants overwhelmingly vote pro-choice.
Nothing the Republican party has done has been able to change this. And take a look at the madly anti-clerical government that has been running Mexico until last week if you think these people put their religion into their politics. It is also an uncomfortable fact, casting doubt on the idea that these immigrants are somehow truly pro-life even if they don't vote that way, that Hispanics in the U.S. have a higher abortion rate than women at large. This fact, in turn, means that simply allowing them into the country, quite independently of any political or population-pressure effects, necessarily drives up the national abortion rate as a matter of simple math. It's about time we did.