Such a reduction, or anything like it, would have huge implications for population and workforce growth; the solvency of Social Security and Medicare; and the Republican Party’s future. Many Republican strategists fear that Trump’s fulminations against undocumented immigrants could alienate Hispanics from the party as lastingly as Barry Goldwater’s opposition to the 1964 Civil Rights Act did African Americans. A parallel campaign to squeeze legal immigration could also repel Asian Americans, who Pew projects will be the largest group of lawful migrants in coming years.

The key to Trump’s new proposal was his call for a commission to develop policies that would, as their first identified goal, “keep immigration levels, measured by population share, within historical norms." Trump’s campaign didn’t respond to requests to clarify how he defined “historical norms.” But Lopez calculates that the foreign-born have comprised on average 10 percent of the U.S. population since 1850.

Today, immigrants represent around 14 percent of the population, well above that average. As Trump correctly noted, the share of the population born abroad is on track “within just a few years” to exceed its all-time high of 14.8 percent in 1890, at the Melting Pot Era’s height. Pew projects that under current law the foreign-born population share will pass that milestone in about a decade—and reach 18 percent by 2065.

Even limiting the foreign-born population share to around 10 percent would still allow considerable numbers of new immigrants to enter over time, both because earlier generations will pass away and the total U.S. population will increase, Lopez notes. But, he quickly adds, not nearly as many would arrive as under current law. As a result, after subtracting deaths and departures, the current U.S. immigrant population of about 45 million would remain unchanged or even shrink in coming decades under Trump’s limits. By contrast, under current trends Pew projects that immigrant population to grow by fully 33 million through 2065. And because future immigrants and their children are projected to provide nearly 90 percent of the total U.S. population growth over that period, “you would be talking about a country that would grow more slowly,” Lopez adds. Over time, that would mean fewer new consumers, homebuyers, and workers.

A smaller workforce would particularly slow the economy’s future expansion. As the Baby Boomers retire, it would also intensify financial pressure on Social Security and Medicare by diminishing the number of working-age adults paying taxes to support each retiree. Reducing legal immigration would mostly reduce “the younger part of the population,” said the Brookings Institution demographer William Frey. “And if you take that out you are making the population older, the elderly dependence ratio higher, and reducing the labor force’s productivity.”