This population explosion myth has been discussed previously in FR also, and IIRC, Paul Ehrlich was a socialist, and his masterpiece Population Bomb has been debunked by many conservatives. (I used to believe that myth before I discovered FR and other conservative sites)
For the sake of this discussion, I will assume that the current US population level is optimum; we still need immigrants to keep the level constant, because white Americans do not want to reproduce at the replacement level.