As I see it US white began as English, Scottish, French, German, Dutch, Belgian and Scandinavian. It has expanded to include Irish, Italian, Russian, Eastern European, Spanish. Not too dark Hispanics are working their way in, as well as, some of the not too dark Asian mixes. White is getting darker and less pinkish. So it is the original iterations of "white" that aren't making enough babies and the new to be white and the next to be white that are growing in population. As long as we keep changing who is white, then we can always have a white majority.
Now of course, those who could be white, but choose to retain their non white heritage just mess things up for white folks.