America’s immigration debate has taken a restrictionist turn. Eight years ago, Donald Trump declared that “when Mexico sends its people, they’re not sending their best,” and promised to build a “big, beautiful wall” on the southern border. That rhetoric, extreme at the time, seems mild now. Today, he depicts immigrants as psychopathic murderers responsible for “poisoning the blood of our country” and claims that he will carry out the “largest deportation operation in the history of our country.”

Democrats have shifted too. In 2020, Joe Biden ran on the promise to reverse Trump’s border policies and expand legal immigration. “If I’m elected president, we’re going to immediately end Trump’s assault on the dignity of immigrant communities,” he said during his speech accepting the Democratic nomination. “We’re going to restore our moral standing in the world and our historic role as a safe haven for refugees and asylum seekers.” That kind of humanitarian language is gone from Democrats’ 2024 messaging. So is any defense of immigration on the merits. When asked about immigration, Vice President Kamala Harris touts her background prosecuting transnational criminal organizations and promises to pass legislation that would “fortify” the southern border.

[Rogé Karma: The truth about immigration and the American worker]

The change in rhetoric did not come out of nowhere. Politicians are responding to one of the most dramatic swings in the history of U.S. public opinion. In 2020, 28 percent of Americans told Gallup that immigration should decrease. Just four years later, that number had risen to 55 percent—the highest level since 2001. (Other surveys find similar results.) Republican attitudes have shifted the most, but Democrats and independents have also soured on immigration.

Although public opinion is known to ebb and flow, a reversal this big, and this fast, is nearly unheard-of. It is the result of a confluence of two powerful factors: a partisan backlash to a Democratic president and a bipartisan reaction to the genuine chaos generated by a historic surge at the border.

Political scientists have long observed that public opinion tends to move in the opposite direction of a sitting president’s rhetoric, priorities, and policies, especially when that president is an especially polarizing figure—a phenomenon known as “thermostatic public opinion.” No president has kicked the thermostat into action quite like Trump. In response to his incendiary anti-immigrant rhetoric and harsh policies, including the Muslim ban and family separation, being pro-immigrant became central to Democratic identity. In 2016, only 30 percent of Democrats told Gallup they wanted to increase immigration; by 2020, that number had grown to 50 percent. In just four years under Trump, Democratic attitudes toward immigration levels warmed more than they had in the previous 15.

But the thermostat works the other way too. When Biden took office, he immediately rescinded many of Trump’s border policies and proposed legislation to “restore humanity and American values to our immigration system.” This triggered a backlash. Right-wing media and Republican politicians sought to turn Biden’s policies into a liability. By mid-2022, the percentage of Republican voters who said immigration should decrease had risen by 21 points. And with Trump no longer in the White House to mobilize the opposition, Democratic immigration attitudes began by some measures to creep closer to their pre-2016 levels as well. “The paradox of Trump was that he inspired an unprecedented positive shift in immigration attitudes,” Alexander Kustov, a political scientist at the University of North Carolina at Charlotte, told me. “But because it was a reaction to Trump himself, that positivity was always extremely fragile.”

Trump is not the entire story, however. Public opinion continued to drift rightward long after Biden took office. From June 2023 to June 2024 alone, the percentage of Democrats who favored decreased immigration jumped by 10 points, and the percentage of Republicans by 15 points. That’s the single largest year-over-year shift in overall immigration attitudes since Gallup began asking the question back in 1965.

[Derek Thompson: Americans are thinking about immigration all wrong]

Voters may have been responding to the sharp rise in so-called border encounters—a euphemism for the apprehension of undocumented immigrants entering the country from Mexico. These reached a record 300,000 in December 2023, up from 160,000 in January of that year and from just 74,000 in December 2020. The surge overwhelmed Customs and Border Patrol, and scenes of overcrowded immigrant-processing centers and sprawling tent encampments became fixtures on conservative media outlets. Texas Governor Greg Abbott began sending busloads of asylum seekers (about 120,000 at this point) to cities such as New York, Chicago, and Denver, which were caught off guard by the influx. Suddenly blue-state cities across the country got a taste of border chaos in the form of stressed social services, migrants sleeping on streets, frantic city officials, and community backlash. “I don’t think the shift in attitudes is surprising, given what’s been happening at the border,” Jeffrey Jones, a senior editor at Gallup, told me. “People are sensitive to what’s going on, and they respond to it.”

Some experts call this the “locus of control theory,” or, more colloquially, the “chaos theory” of immigration sentiment. The basic idea, grounded in both survey data and political-science research, is that when the immigration process is perceived as fair and orderly, voters are more likely to tolerate it. When it is perceived as out of control and unfair—perhaps due to an uncommonly large surge of migrants—then the public quickly turns against it. Perhaps the best evidence for this theory is that even as Americans have embraced much tighter immigration restrictions, their answers to survey questions such as “Do you believe undocumented immigrants make a contribution to society?” and “Do you support a path to citizenship for undocumented immigrants?” and even “Should it be easier to immigrate to the U.S?” haven’t changed nearly as much, and remain more pro-immigrant than they were as recently as 2016. “I don’t think these views are contradictory,” Natalia Banulescu-Bogdan, a deputy director at the Migration Policy Institute, told me. “People can simultaneously have compassion for immigrants while also feeling anxious and upset about the process for coming into the country.”

One implication of chaos theory is that leaders can mitigate opposition to immigration by introducing reforms that make the process less chaotic. That’s what the Biden administration tried to do in June of this year, when it issued a series of executive orders that would, among other things, bar migrants who cross illegally from claiming asylum and give the Department of Homeland Security the ability to halt the processing of asylum claims altogether if the volume of requests gets too high. Border encounters have fallen steadily throughout 2024, reaching about 100,000 in July and August—still a high number, but the lowest level since February 2021. Perhaps not coincidentally, the salience of immigration for voters has also been falling. This past February, 28 percent of Americans told Gallup that immigration was the most important problem facing the country; by August, that number had dropped to 19 percent. (It crept back up to 22 percent in September, for reasons that likely have more to do with the wave of disinformation about Haitian migrants than with crossings at the border, which continued to fall.)

The very fact that Biden had to rely on unilateral executive orders, which are being challenged in court, illustrates a deeper issue. Even though most Americans want a more orderly and fair immigration system, the nature of thermostatic public opinion gives the opposition party strong incentives to thwart any action that might deliver it. Earlier this year, congressional Republicans killed a border-security bill—which had previously had bipartisan support—after Trump came out against it, lest the Biden administration be given credit for solving the issue that Trump has staked his campaign on. And if Trump is reelected, the pendulum of public opinion could very well swing back the other way, putting pressure on Democrats to oppose his entire immigration agenda.

What’s clear is that the current hawkish national mood is not the fixed end point of American popular sentiment. Attitudes toward immigration will continue to fluctuate in the years to come. Whether public policy changes meaningfully in response is anyone’s guess.