Although few Republicans represent places with large immigrant populations, in recent years most GOP legislators have hesitated about opposing legal immigration.

The last serious conservative push to reduce it came in 1996—and it failed decisively despite Republican control of both Congressional chambers. Though restrictions cleared the House Judiciary Committee, one-third of House Republicans joined most Democrats to strip them on the floor. In the Senate, legal immigration reductions attracted just 20 votes, drawing opposition from almost all Democrats but also nearly three-fourths of Republicans-including John McCain, Orrin Hatch and Mitch McConnell.

Cotton and Perdue would squeeze legal immigration even more tightly than the GOP proposed then. Based on preliminary estimates, the Pew Research Center projects that limiting future immigrants and refugees to about half their current level would, over roughly the next 50 years, reduce the foreign-born share of the population to just below 10 percent. That's lower than at most points in American history, and significantly below today’s level of around 14 percent. Such a diminished share, however, would track Trump's campaign call to limit future immigration flows to a level that would return the foreign-born population share to what he termed "historical norms"––which has averaged about 10 percent since the Civil War, according to Mark Hugo Lopez, Pew's director of Hispanic research.

Pew’s projections also suggest that with immigration levels like those envisioned by Cotton and Perdue, the number of working-age Americans in coming decades would remain essentially unchanged at around 175 million. By contrast, under current law and migration trends, Pew Research projects the number of working-age Americans will increase by about 30 million over the next 50 years, with immigrants and their descendants contributing almost all of the increase.

Cotton and Perdue say preempting that growth would increase economic opportunities for native-born workers, and some economists agree—to a point. In an exhaustive study last fall, the National Academy of Sciences found “little evidence” that increased immigration significantly affected employment levels for native-born workers, but did see indications it has pressured wages for lower-skilled workers––primarily recent immigrants themselves but also native-born workers who didn’t finish high school (a slim share of the overall workforce). But even there, the study concluded immigration’s impact on wages for native workers “is very small…when measured over a period of 10 years or more.”

Any benefit that might derive from squeezing immigration to benefit those workers would carry other costs. Smaller workforce growth would mean lower overall economic growth. Fewer workers also threaten Social Security and Medicare. The Social Security trustees estimate the senior population will soar from about 48 million now to 86 million in 2050. Without more workers, the taxes needed to support those retirees could reach unsustainable levels, increasing pressure for benefit cuts. Immigration helps maintain a more sustainable balance between the working age and retired population, especially because a significantly higher share of foreign-born adults (half) than native-born (one-third) are younger than 45.