One of the basic rules of politics is to define issues in a way that maximizes the number of voters on your side and minimizes the number of voters on the other side. That’s why prudent Republicans frame the immigration debate around lawbreaking. They praise legal immigration, and they claim to care about all Americans who could lose their jobs to border jumpers. The issue isn’t about ethnicity, they argue. It isn’t even about whether you were born here. It’s about respecting the law and getting in line. That’s a message most Americans can support.

President Donald Trump has wrecked that message. This week, he told four Democratic congresswomen to “go back” to the countries from which their families emigrated. Three of the four lawmakers—Alexandria Ocasio-Cortez of New York, Ayanna Pressley of Massachusetts, and Rashida Tlaib of Michigan—were born here. By targeting them, Trump has sent a warning to every American whose ancestors came to this country: You, too, can be told to go home.

Trump has often complained about minority immigrants, regardless of their legal status. Go back and watch the 2015 speech in which he kicked off his presidential campaign. “When Mexico sends its people, they’re not sending their best,” he declared. “They’re bringing drugs. They’re bringing crime. They’re rapists. And some, I assume, are good people.” Not once in that speech did Trump specify whether he was talking about legal or illegal immigrants.

In public, Trump sometimes pretends to love legal immigrants. But behind closed doors, he has made clear that he doesn’t like people of certain nationalities. In 2017, during an Oval Office meeting about legal immigration, Trump claimed that Haitians coming to the United States “all have AIDS” and that Nigerians, if allowed into our country, would never “go back to their huts.” In 2018, during another conversation about legal immigration, he said of Haiti, El Salvador, and Africa: “Why are we having all these people from shithole countries come here?” He groused on Twitter about having to accept “people from high crime countries which are doing badly.”

Opposing immigrants based on their race, ethnicity, or nationality is far more incendiary than opposing immigrants who break the law. It’s also politically dangerous. It risks antagonizing not only the many Americans whose families came from Africa, the Middle East, or Latin America, but also millions of other voters who don’t feel comfortable with overt prejudice. Still, Republicans might get away with it—and profit from the votes of xenophobes and bigots—as long as they don’t take the next step. The next step is using ethnicity to target not just immigrants, but natural-born American citizens.

Trump has crossed that line many times. In 2015, he impugned the patriotism of Jeb Bush, the former governor of Florida, for having married an immigrant from Mexico. Bush’s wife, Columba, has been an American citizen for nearly 40 years. That didn’t stop Trump from suggesting, based on Columba Bush’s ancestry, that Jeb Bush couldn’t be trusted to secure America’s borders. First Trump retweeted an allegation that Jeb Bush “has to like the Mexican illegals because of his wife.” Then, on CNN, Trump repeated that allegation: “If my wife were from Mexico, I think I would have a soft spot for people from Mexico.”

Trump also challenged the faith and loyalty of Sen. Ted Cruz of Texas, an American citizen whose father had immigrated legally from Cuba. During the 2016 presidential race, Trump persistently warned evangelicals not to trust Cruz because, as Trump put it, “not a lot of evangelicals come out of Cuba.” In addition, Trump disputed Cruz’s eligibility for the presidency on the grounds that Cruz “was born in Canada.” Cruz was automatically American and eligible to serve because his mother was a U.S. citizen. But to Trump, that didn’t matter.

Sometimes, in the course of vilifying ethnic communities, Trump doesn’t bother to clarify whether he’s talking about noncitizen immigrants, naturalized Americans, or people who were born here. In 2015, at a rally in Alabama, he falsely claimed to have seen “thousands of people” celebrating the 9/11 attacks in “New Jersey, where you have large Arab populations. They were cheering as the World Trade Center came down.” In case anyone missed the slur, Trump repeated it: “There were people over in New Jersey that were watching it, a heavy Arab population, that were cheering as the buildings came down.” The size of Trump’s allegation—“thousands of people,” “large Arab populations”—implied that he was accusing Arab Americans, not just immigrants, of supporting anti-American terrorism.

Sometimes Trump explicitly targets natural-born Americans of certain ethnicities or religions. In 2016, he warned of future terrorism by Americans whose parents came “from the Middle East.” “The children of Muslim immigrant parents—they’re responsible for a growing number … of terrorist attacks,” Trump declared at a rally in North Carolina. In a Fox News interview, he claimed that among “second- and third-generation” American Muslims, “there’s no real assimilation.” In a prepared speech, he decried terrorism by “recent immigrants and their children.”

This week’s attack on the Democratic congresswomen marks an escalation of Trump’s rhetoric against minority immigrants and their descendants. In a series of tweets, Trump treated the four lawmakers as foreigners. They “originally came from countries whose governments are a complete and total catastrophe,” the president wrote, and instead of “telling the people of the United States … how our government is to be run,” they should “go back and help fix the totally broken and crime infested places from which they came.” When reporters reminded Trump that all four women were American citizens and that three were born here, he retorted, “They have to love our country.” That’s a telling phrase. In his 2016 campaign, Trump used it repeatedly to describe immigrants: “We want to take people in. … But they have to love our country.” Now Trump is applying the same scare talk to American-born descendants of immigrants.

Many Republicans, despite their personal or political misgivings about Trump’s bigotry, argue that it doesn’t matter. He might smear Latinos, Africans, or Muslims now and then, they reason, but he doesn’t really do anything about it. But Trump has attempted, committed, or defended specific legal acts against natural-born Americans based on their ancestry. In 2015, he proposed a “complete shutdown of Muslims entering the United States,” without regard to citizenship. Last year he proposed to repeal birthright citizenship, the constitutional rule that says anyone “born or naturalized in the United States” under American jurisdiction is a citizen. In 2017, he pardoned Joe Arpaio, a former sheriff who defied a court order to stop using ethnic profiling to detain people in Arizona whom Arpaio suspected of being here illegally. Under the Trump-Arpaio rule, you don’t have to be an illegal immigrant to be detained. You just have to look like one.

This isn’t about immigration. It’s about dividing natural-born Americans by race, ethnicity, and religion.

For years, Trump directed and publicized an investigation of President Barack Obama, insinuating that Obama was a Muslim “born in Kenya” and not, as all records showed, a Christian born to an American mother and a Kenyan father in the United States. Trump didn’t just question whether Obama “was born in this country.” He hired investigators to scour records from Obama’s birth and childhood, and he claimed on TV that the investigators “cannot believe what they’re finding.” After Obama released his birth certificate, Trump tweeted, “An ‘extremely credible source’ has called my office and told me that @BarackObama’s birth certificate is a fraud.” Later, in a tweet that foreshadowed his plea for Russian intervention in the 2016 election, Trump urged “hackers” to pursue the question of Obama’s birthplace.

In addition to his investigation of Obama, his defense of ethnic profiling, and his attempt to repeal birthright citizenship, Trump has advocated overt discrimination against public officials. In 2016, he denounced Gonzalo Curiel, a federal judge who was born in Indiana to legal immigrants from Mexico. Based on Curiel’s ancestry—and the fact that Trump was advocating a crackdown on Mexican immigration—Trump demanded that the judge “recuse himself” from presiding over a fraud lawsuit against Trump University. Trump summarized his argument this way: “We’re building a wall. He’s a Mexican.” In another interview, Trump argued that Curiel’s “Mexican heritage” presented “an inherent conflict of interest.”

Trump has also proposed surveillance and punishment of American Muslims. In 2015, he was asked about a hypothetical “database that tracks the Muslims here in this country.” He replied, “There should be a lot of systems, beyond databases.” Later, Trump explained that he meant “surveillance, including a watch list.” When he was asked about “registering Muslims in a database or giving them a form of special identification that noted their religion,” he answered, “We’re going to have to look at a lot of things very closely. We’re going to have to look at the mosques.” At a campaign rally, Trump said of a hypothetical Muslim registry, “Database is OK, and watch list is OK, and surveillance is OK. … I want to surveil.” In a Fox News interview, he argued that Muslims in the United States, without regard to their citizenship, should be punished collectively for terrorism committed by other Muslims. “The Muslims are the ones that have to report” plotters, said Trump. “And if they don’t report them, then there have to be consequences to them.”

Trump’s history of attacks on Americans of African, Middle Eastern, and Latin American ancestry—impugning their loyalty, investigating their births, threatening their citizenship, proposing to punish them collectively—isn’t about respecting immigration laws. It isn’t even about immigration. It’s about dividing natural-born Americans by race, ethnicity, and religion. His denunciation of members of Congress based on where their families “originally came from” signals an acceleration of that assault. It might galvanize Trump’s white supporters. On the other hand, it might alarm and galvanize many of us, white or nonwhite, whose parents or grandparents came from other countries. We might decide that a racist president is a threat to us all.