With or without immigration, the White share of the population will decline in the coming decades, census projections show. But if immigration is reduced or eliminated, America will grow older, with many fewer working-age adults available to support an exploding number of retirees.And that would not only slow overall economic growth, multiple projections have found, but also would increase pressure for cuts in the Social Security and Medicare benefits that provide a lifeline to the older Whites most drawn to the right’s anti-immigrant arguments.