At last night’s Democratic debate, Elizabeth Warren stated, “A great nation does not separate children from their families.” It was a forceful rejoinder to many of the Trump administration’s immigration policies: separating more than 5,400 children from their families, sending families to Mexico to await the chance to make asylum claims and seeking the chance to detain children indefinitely. The administration has also supported efforts that would end “chain” migration and de-prioritize family reunification.

When discussing immigration, candidates like Warren are likely to gain support when they speak in support of keeping families together. Making family reunification the centerpiece of the legal immigration system strengthened the United States in the 20th century while increasing the migration of people of color and benefiting U.S. citizens who were able to bring family members to join them. Keeping families apart is an effort to reduce immigration and whiten America — and history suggests that despite attempts by nativists to brand family reunification with the derisive, dehumanizing term “chain” migration, the public will remain steadfastly supportive of keeping families together.

Around the turn of the 20th century, many families immigrated together to the United States. Between 1871 and the outbreak of World War I, 12.9 million immigrants came from Asia and Europe in search of economic opportunities, social mobility or safety. They joined millions of migrants around the globe who, at the end of the 19th century, left their countries to escape stagnant economies, political unrest or persecution to take advantage of the demand for unskilled labor in rapidly industrializing nations.

By 1920, the three largest groups of immigrants in the United States were Italians (4 million), Eastern European Jews (2 million, mostly from the Russian Empire) and Poles (1 million). These numbers stood in stark contrast to the years preceding the mass migration of the turn of the 20th century. Until the 1880s, 11,725 Italians and about 150,000 Jews, mostly of German descent, had entered the United States.

These new immigrants faced discrimination in the United States. Many Americans of Northern and Western European ancestry regarded them as nonwhite, biologically and culturally inferior and unassimilable. Calls for immigration restrictions against Asian and European immigrants echoed a global push to restrict, exclude, deport and segregate immigrants deemed “undesirable” — especially immigrants of color.

As the debate raged, Italian and Jewish leaders skillfully put family ties at the forefront of their efforts to staunch the nativist push. Starting in 1896, when Congress first considered administering a literacy test to incoming immigrants — a tactic to reduce immigration — Italian and Jewish reform advocates argued that many of the proposals under consideration in Congress would lead to family separation. For the moment, framing restriction as harming families worked, and the bills failed to make their way into law.

But a few years later, as large-scale immigration of families continued, nativist opponents of immigration demanded policy change. Soon, immigration laws emerged as the ideal tool of social engineering and nation building. As Prescott Hall, one of the leaders of the Immigration Restriction League, put it in 1919, “immigration restriction is a species of segregation on a large scale, by which inferior stocks can be prevented from both diluting and supplanting good stocks.” Framing immigrants as “stocks” diluting white America originated from popular eugenicist beliefs that immigrants and their families posed a threat to the survival of U.S. society through reproduction.

As pressure to close the gates mounted, Italian and Jewish reform advocates joined other groups to oppose restrictive immigration laws. They pushed to exempt family members from the quota system under discussion, and advocated for family reunification. But their efforts collided with those of a powerful coalition of nativists in Congress who worked to solidify a regime of restriction and strengthen their political influence.

When Congress passed the 1924 Immigration Act, which imposed a near ban on immigration from Asia and restricted immigration from Europe through the national origins quota system, Rep. Albert Johnson (R-Wash.), one of its sponsors, was clear about the motivation. He called it “America’s second declaration of Independence” and argued that “the United States is our land … if it was not the land of our fathers, at least it may be, and it should be, the land of our children. We intend to maintain it so. The day of unalloyed welcome to all peoples, the day of indiscriminate acceptance of all races, has definitely ended.” Aiming to whiten the country — to ensure that American families remained white — meant legally restricting the immigration of people deemed nonwhite.

But over the course of the 20th century, reformers sought to open the gates that had been shut so tightly. Gradually appeals to American family values emerged as the main tool to challenge restriction. Italian and Jewish reformers understood that, despite their differences, congressional leaders of both parties were willing to negotiate over family reunification as an exception to restriction because many of them regarded the family unit as the foundation for U.S. society.

Their efforts finally began to succeed in the 1950s. Although Italian and Jewish reform advocates were frustrated when the 1952 Immigration and Nationality Act retained the national origins quota system from the 1920s, they welcomed the law’s new emphasis on family reunion.

Because of the act’s emphasis on family reunification, education and economic potential, immigration increased in the late 1950s. Immigrants from outside of Europe took advantage of these new family provisions, despite the small annual quotas allocated to their countries. In turn, these trends paved the way for a more diverse society.

Under the law, more than 2 million immigrants should have arrived between 1952 and 1965, but in fact 3.5 million people entered the country, only about a third of whom came under the annual immigrant quotas allocated by the law. Many instead entered through the family reunion provisions. This trend expanded considerably after Congress, again embracing the centrality of family, passed the 1965 Immigration and Nationality Act, which replaced the national origins quota system with a global ceiling on immigration and retained the emphasis on family reunion, skills and education.

Restrictionist legislators’ support for family reunion in 1952 and 1965 was no accident. Their decision to prioritize family reunion was rooted in their belief that emphasizing family would be the best way to preserve the existing racial status quo. They expected that, because the 1920s legislation prioritized Northern and Western Europeans, that they would be the perpetual beneficiaries of family-based migration.

But the rapid postwar recovery of Western European nations, greatly facilitated by the Marshall Plan, meant that citizens of these countries no longer sought to emigrate in large numbers, while totalitarian governments in eastern and central Europe blocked aspiring migrants from leaving altogether. Meanwhile, the pressure to leave increased in Latin America, Asia and Africa.

Restrictionists never expected that instead of preserving whiteness, family preference would become a leading factor in creating the most diverse society in U.S. history.

Today’s efforts to undermine family reunion as the centerpiece of U.S. immigration policy and the use of family separation as a deterrent to immigration are not by chance. In fact, as restrictionists have come to recognize their miscalculation and that family reunion has benefited immigrants of color, they have adjusted their tactics to attempt to preserve the racial status quo.

But the public has shown little support for the Trump administration’s policies targeting families. Like the mid-century reformers who pushed family reunion to the center of immigration policy, people today continue to embrace the family unit as the foundation for our society. The question for the future of U.S. immigration, in 2020 and beyond, will be whether preserving families or preserving the racial status quo is the more important priority.