Here's a good analysis by Pew which shows that even if the U.S. closed its border today, we would still see amazing growth in the Mexican-American population since their age structure is so young, and since they have so many babies--especially immigrant parents. (They don't mention shorter generations, but that is part of the story as well). (Just last night, I took my wife to the ER for a kidney stone and sat next to a Mex-Am mom with six kids. Don't tell me a little Hispanic women has no power.)
The authors project that, based on current trends, Hispanics will be 29 percent of the total population by mid-century. That is more than double the size of any other ethnic group. Add blacks and Jews, and you're--what--about 45 percent of the country. That means that conservatives will have to get the vote of just about every non-Jewish white person to win elections. Given the white proclivity for self-destruction, good luck with that.