Donald Trump first gained political prominence in 2011 by peddling the “birther” conspiracy theory that Barack Obama was not born in the United States. Trump is now trying to repeat that strategy by questioning the citizenship of presumptive Democratic vice-presidential nominee Kamala Harris. This time around, the fringe theory on offer is not that Harris was born in another country, but that her birth in the United States — in Oakland, California, in 1964 — is not enough to make her a citizen because her parents were immigrants who may have been present on temporary visas at that time.As a legal matter, the claim is laughable: both longstanding Supreme Court precedent and well-established federal policies recognise that a person born in the United States is a citizen under the Fourteenth Amendment to the US Constitution. But to get caught up in the legal doctrine is to miss the point: in questioning the citizenship of Harris — the first woman of colour on a major party ticket and the second woman of colour ever to run for vice-president, after Charlotta Bass in 1952 — Trump is once again reaching into the traditional playbook of American white supremacy. Throughout the country’s history, debates over citizenship have always been deeply entwined with racialised notions of who should be considered a “real” American.In 1857, in dismissing a lawsuit brought by Dred Scott to win his freedom from slavery, the Supreme Court held that black people, whether enslaved or free, were excluded from citizenship because the founders had intended to erect a “perpetual and impassable barrier ... between the white race and the one which they had reduced to slavery”.That infamous decision was overturned after the Civil War by the adoption of the Fourteenth Amendment, which declared that “(all) persons born or naturalised in the United States, and subject to the jurisdiction thereof, are citizens of the United States and of the State wherein they reside”. These words made birthright citizenship unrestricted by race for the first time in the nation’s history, at least as a matter of formal doctrine. (The struggle to make this promise of citizenship truly meaningful is continuing, as movements such as Black Lives Matter show us.)Even after the ratification of the Fourteenth Amendment, however, Americans continued to argue about race and birthright citizenship. Less than two decades later, government officials and some legal scholars contended that the American-born children of Chinese immigrants did not qualify as birthright citizens because their parents, as Chinese citizens who themselves were racially barred from naturalising, were not “subject to the jurisdiction” of the United States. (Naturalisation, in contrast to birthright citizenship, remained racially restricted until 1952.)These arguments arose at a time when America was in the grips of an ugly backlash against Chinese immigrants. Congress had barred almost all emigration from China through the Chinese Exclusion Act of 1882. Many of the arguments in favour of excluding Chinese-Americans from birthright citizenship rested on the notion that Asian-Americans were racially unfit to be American citizens.The Supreme Court flatly rejected these arguments in Wong Kim Ark in 1898, holding that constitutional birthright citizenship encompassed all those born within the United States, with the exception of children born to foreign diplomats, members of invading armies or members of Native American nations.But the arguing continued, with the quest for racial restrictions on birthright citizenship soon becoming a staple of American nativism. This was especially evident within organisations focused on excluding Japanese immigrants and their children in the early 20th century. When the American Legion was founded in 1919, one of its first actions was to pass a resolution supporting a constitutional amendment to exclude Asian-Americans from birthright citizenship. Several members of Congress introduced such amendments in the 1920s but never managed to win over a majority of their colleagues.In 1942, amid the shameful episode of America’s forcible internment of both Japanese immigrants and Japanese-Americans as members of an “enemy race”, California nativists went to court in an attempt to compel the San Francisco registrar of voters to remove Japanese-Americans from voter rolls. They trotted out 19th-century legal arguments about Asian-Americans not being entitled to birthright citizenship. The court quickly dismissed their claim, finding that the law on the matter was clearly settled.Since the 1980s, arguments for restricting birthright citizenship have focused almost entirely on trying to exclude the children of undocumented immigrants. Contemporary white nationalists often speak of such children — who, in the nativist imagination, are always of Mexican descent — as representing an “invasion” from Mexico. Their framing mirrors that of their counterparts a century ago, who described Japanese-American children as representing an “invasion” from Japan.Over the past three decades, Republican-elected officials have repeatedly proposed amending the Constitution to impose limits on birthright citizenship, without any more success than nativist politicians in the 1920s. And a handful of legal scholars have continued to try to revive old legal theories in support of a narrow reading of the citizenship clause.None of these efforts has ever gained traction in the courts or in Congress. So why do they persist? Because the question of who belongs remains at the heart of our political discourse. How do we imagine ourselves as a nation? White Americans have always been more divided on that question than some would like to admit — and have arguably become more divided over the past four years, under a president who has sought to exploit such divisions. Efforts to narrow birthright citizenship are expressions, either explicit or implicit, of a desire to preserve a racial order that upholds white supremacy.Kamala Harris — black, Indian-American and the daughter of immigrants — is the latest canvas on which Americans are projecting both their hopes and their fears about what it means to be an American. Harris’s background is an inspiration to many, and her citizenship is beyond question. But as long as white Americans remain divided about whether people of colour are fully American, we can unfortunately expect debates about birthright citizenship, however much on the fringes, to remain an enduring element of the political landscape.• Rachel E. Rosenbloom is a professor of law at Northeastern University School of Law. She is writing a book on the history of constitutional birthright citizenship