So, the fear is that there are fewer future whites and the non-whites are dem-leaning?
But isn't the fewer future whites part of a universal all-color demographic trend where a society becomes well-off materially, deliberately limits offspring to better enjoy material benefits, becomes career-focused that causes further birthing constraints, and ends up as an ageing one with a net negative population growth? This has been seen across early movers such as Japan and Europe and later ones such as China (despite childbirth limits relaxation). So, if whites want to have children, just have them, I guess?
But if the argument is really about voting blocks manipulation through immigration, I think there is enough evidence to show that naturalized citizens are roughly 50-50 between the two parties. I remember when Muslims voted heavily in favor of Bush Jr. in 2000 and believe they were the difference in him winning (voted heavily for Bernie the last two dem primaries because Trump absolutely scared them, not because they didn't believe in GOP ideals preferred D over R). Also, many immigrants that I know instinctively align with small government, self-help, traditional family, and wrecking the school grade curve; supposedly right in line with classical conservatism. One may say that the culture among young or at universities is shifting to the left, but that is independent of immigration.