The Trump administration is currently finalizing a plan to prevent documented, “legal” immigrants who have received any public assistance from attaining citizenship.

This policy change would affect millions of people who are already in the country and have legally benefitted from programs including but not limited to the Affordable Care Act, Social Security and the Supplemental Nutrition Assistance Program.

And yet, this change wouldn’t require any new laws being passed. Because our immigration laws, including the public charge clause of the Immigration Act of 1917, afford the executive branch enormous discretion, the president has the power to decide how the law is used.

But public opinion also matters. When members of Franklin Roosevelt’s Cabinet attempted to use this provision to help Jewish Germans fleeing Nazi territory enter the United States, their plan ultimately floundered because of the national mood, which was infused by post-WWI isolationism and xenophobia. This offers a stark reminder that the American people can shape the outcome of President Trump’s immigration proposals, no matter how harsh, if they mobilize and make clear that imposing draconian immigration policies will cost Trump politically and crush the remainder of his agenda.

Through the public charge clause, the Immigration Act of 1917 enabled American citizens — namely family members and organizations — to offer a “charge bond” of approximately $500 to support an immigrant who was “likely to become a public charge,” usually for reasons of financial hardship or physical disability. The government was concerned about welcoming immigrants in need of financial or other assistance, and charge bonds provided a safety net so the federal government wouldn’t have to.

In March 1933, Roosevelt’s Cabinet convened two months after Germany had elected Adolf Hitler, to discuss the sudden influx of immigration applications from Jewish Germans. The State Department, dominated by anti-immigration conservatives, aimed to assert itself and control the administration’s handling of the situation. As such, the under-secretary of state called Secretary of Labor Frances Perkins — the first woman Cabinet secretary — to instruct her regarding quotas and visas. Perkins rebuffed his instructions, reminding him that the Immigration and Naturalization Service (INS) remained in the Department of Labor and thus under her control.

Perkins had a different vision: Future Supreme Court Justice Felix Frankfurter suggested she use the public charge clause to allow charge bonds for Jews fleeing Nazi territory. This provision would then assuage concerns that many Americans had about an influx of immigrants in the face of the Depression and would therefore enable the United States to admit entry to more people fleeing Nazi territory. Perkins relayed this option to Roosevelt. The Department of State immediately objected, arguing that such a usage of charge bonds defied the “spirit” of American immigration law.

This ignited a turf war between the Departments of Labor and State, deeply molded by cultural and political differences between the two. The State Department was institutionally racist, while Perkins’s Labor Department was comprised of New Dealers and progressives committed to aiding people in need as much as possible, regardless of national origin.

Roosevelt instructed Perkins and Secretary of State Cordell Hull to submit legal opinions on each side of the charge bonds issue. Perkins’s Solicitor of Labor Charles Wyzanski — a Jewish American with family trapped in Nazi territory — drafted the Labor Department’s position. In late 1933, Attorney General Homer Stiles Cummings sided with Wyzanski, Perkins and the legality of charge bonds.

And yet the administration never implemented the charge bond proposal. The archival record does not offer conclusive evidence as to what happened, but a backdoor negotiation seems to have commenced after Cummings handed down his decree. It seems likely that Perkins and her department bartered away the charge bond option in exchange for support for other, higher priorities.

This was necessary because, while social reformers in the New Deal coalition believed that immigration boosted the economy, they ran into skepticism from organized labor. Representing American workers, unions such as the American Federation of Labor feared that immigrants would threaten union members’ opportunities for advancement. And support from these unions was integral to passing New Deal programs through Congress, which meant that the Roosevelt administration needed to keep them happy.

These politics doomed the charge bond proposal, which might have significantly aided the masses fleeing Nazi territory in the early 1930s.

Now the public charge clause is back in the news. Ironically, some of the same assistance programs that Perkins’s Department of Labor prioritized over the charge bond option, such as Social Security, have catapulted it back onto the front pages .

Trump aide Stephen Miller wants to use the public charge clause to restrict immigration and deport documented immigrants to their home countries, while also undermining the goals of social welfare programs like Social Security and even the subsidies provided to help Americans buy health care under the Affordable Care Act. These entitlements offered to all citizens are now being used against immigrants who consider America to be their only home.

While Perkins’s effort in the 1930s and Miller’s effort today have starkly different goals and motivations, both are possible because American immigration law has gray areas that afford opportunities for presidents to skew policy as they wish. Presidents, including Roosevelt, Lyndon Johnson and Barack Obama, have used this discretion to move immigration policy in a more liberal, welcoming direction. But immigration advocates are learning that the same legislative language that they’ve capitalized on in the past affords President Trump the opportunity to impose draconian, restrictionist policies.

If the Trump administration successfully wields the public charge clause against documented immigrants, they would be using this provision to resurrect the very restrictionist immigration policies from the interwar years that Perkins and her allies attempted to challenge. And they’d be doing it despite decades of subsequent laws that opened up and liberalized immigration policy.

The gray area in American immigration law means that the same legal provisions can be mobilized to protect vulnerable migrants or to stigmatize and subject them to violence.

But as Perkins learned, there is a brake on executive discretion in immigration policy: public opinion. Perkins could not aid people as she pleased because national sentiment and the mobilization of interest groups leaned elsewhere.

Whether the Trump administration can proceed without significant obstacles will be determined by where the American public stands today. If Americans rise up against this proposal, and inflict political damage, they may thwart it. If, however, the public acquiesces, implementation of the plan will be more a reflection of a return to anti-immigrant sentiment than an administration twisting the law.