With so many other confrontations over immigration already raging, it was easy to overlook that new skirmish that Republican Senators Tom Cotton of Arkansas and David Perdue of Georgia started last week.

Just weeks into office, President Trump is embroiled in legal and political struggles over his contested travel ban on seven Muslim-majority nations and the expanded criteria for deporting undocumented migrants his administration finalized this week. Cotton and Perdue opened a new front in these escalating immigration wars by proposing legislation that would cut in half the number of legal immigrants and refugees allowed into the U.S. from today’s combined level of about 1.1 million annually. Echoing Trump, Cotton insisted that high immigration levels undermined wages for working-class Americans and threatened to leave them as “a near permanent underclass.”

The bill faces a steep hill because it’s unlikely to attract enough Democrats to break a Senate filibuster. But the legislation will still measure how many Congressional Republicans are embracing Trump’s effort to redefine their party around a bristling defensive nationalism––and how many Congressional Democrats feel compelled to join them.

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.

A version of that dynamic is already evident in communities across the Rustbelt states central to Trump’s electoral success. Cities of all sizes there have been actively recruiting legal immigrants to combat population decline and replenish their workforce. In an important study for the Chicago Council on Global Affairs, public policy consultant Rob Paral found that the native-born working age population declined from 2000-2010 in dozens of Rustbelt cities––from Detroit, St. Louis and Kansas City to Akron and Sheboygan––as families left for faster-growing regions. Those places all mitigated that crippling contraction by increasing their population of working-age foreign-born adults.

Although many whites across the region view large-scale immigration as an economic or cultural threat, in fact, “In the Midwest and the Northeast virtually all the metro areas have become dependent on immigration,” Paral says. “Immigration cuts would pull the rug out from under them.”

Like Trump’s protectionist trade agenda, the Cotton/Perdue immigration restrictions envision a zero-sum economy, in which any gains for foreign interests mean losses for domestic workers. That contrasts with the dynamic vision of mutual benefit that an alliance of mostly technology companies-including Apple, Google, Intel and Facebook-presented in their legal brief opposing Trump’s seven-nation travel ban.

“Immigrants make many of the Nation’s greatest discoveries, and create some of the country’s most innovative and iconic companies,” the companies wrote. “The energy they bring…is a key reason why the American economy has been the greatest engine of prosperity and innovation in history.”

In the Trump era, the chasm is quickly widening between those who believe the best way to ensure America’s prosperity (and security) is to build bridges to the world––and those who are determined to erect walls against it.