Breaking news: Baltimore has rats and vacant homes! On Saturday, President Donald Trump blamed West Baltimore’s many woes on Maryland Rep. Elijah Cummings, writing in a racist Twitter thread that the Democratic leader’s district is “disgusting” and a “rat and rodent infested mess … very dangerous … filthy.” Trump appeared to be reacting to Cummings’ criticism of conditions at U.S. Customs and Border Control facilities at the Mexican border—and apparently to a Fox News segment containing footage of trash and vacant houses in West Baltimore. Trump called the district a place where “no human being would want to live,” which is explicit even for Trump when it comes to dehumanizing black people.

Rep, Elijah Cummings has been a brutal bully, shouting and screaming at the great men & women of Border Patrol about conditions at the Southern Border, when actually his Baltimore district is FAR WORSE and more dangerous. His district is considered the Worst in the USA...... — Donald J. Trump (@realDonaldTrump) July 27, 2019

....As proven last week during a Congressional tour, the Border is clean, efficient & well run, just very crowded. Cumming District is a disgusting, rat and rodent infested mess. If he spent more time in Baltimore, maybe he could help clean up this very dangerous & filthy place — Donald J. Trump (@realDonaldTrump) July 27, 2019

Why is so much money sent to the Elijah Cummings district when it is considered the worst run and most dangerous anywhere in the United States. No human being would want to live there. Where is all this money going? How much is stolen? Investigate this corrupt mess immediately! — Donald J. Trump (@realDonaldTrump) July 27, 2019

Whether Trump’s tweets were an impulsive, racist act or an attempt to turn conversation away from talk of impeachment following last week’s Mueller testimony—or, more likely, some combination of both—they were, in a way, instructive. One method Trump uses to appeal to his base is talking about, and attempting to erect, borders. It’s what he is doing in the Southwest United States, and it’s what he is doing by talking about West Baltimore. Baltimoreans know all about this tactic: Pick a nonwhite face, associate it with a nonwhite place, and demonize them both. (Trump conveniently ignored the rest of Cummings’ district, which includes wealthy, white communities in Howard and Baltimore counties.)

The language of demonization that Trump and his supporters use often pales in comparison with language used for a century to oppose public and low-income housing in white communities.

Harvard sociologist Michèle Lamont has a term for what Trump is doing: boundary work. Lamont and two colleagues studied Trump’s electoral speeches and found him drawing what they call “symbolic boundaries” around white workers. First, he absolved them, blaming their problems on globalization. Then he pitted them against elites above them and minorities and immigrants below them. Symbolic boundaries work because of the way in-groups compare themselves with out-groups to construct their collective identity. In-groups construct stereotypes of the out-group’s inferiority that help the in-group maintain its perceived superiority.

Lamont found that Trump’s speeches celebrated the virtues of white workers while characterizing immigrants as threatening and associating black Americans with poverty and violence. He “constantly referred to the groups as ‘they,’ as if to separate them from broader society,” Lamont wrote. Trump’s boundary work has continued in office, as seen in his preference for hypothetical Norwegian immigrants and his disdain for “shithole countries” and the nonwhite Democratic U.S. representatives he suggested should “go back” to other countries.

Lamont argues that symbolic boundaries help generate “social boundaries,” the real-life forces that create disparities in access to resources and opportunity. This is how, Lamont writes, “mechanisms of exclusion at the micro level translate into broader patterns of inequality.” Often, those forces take the form of policies and laws. When Lamont studied Trump’s speeches, she found his “boundary work was accentuated by his deep concern for physical/spatial boundaries,” most obvious in his fervor for a wall at the Mexican border.

Trump’s boundary work goes back decades. In the 1970s, Trump and his father fought a Justice Department suit for two years over claims that their company rented units to white families after telling black renters that the units weren’t available. They never admitted guilt, but they ultimately signed a consent order ordering them to take steps to ensure fair housing.

How language choices can lead to physical barriers is a topic familiar to Baltimore. Residents in our suburbs and our few white city neighborhoods have been doing boundary work around real estate for decades. Before Trump worried about an invasion from Central America, northern cities fought a perceived invasion of black Americans fleeing exploitation and brutality in the South. The language of demonization that Trump and his supporters use when talking about a border wall—“invasion,” “drugs,” “criminals,” “gangs”—echoes and often pales in comparison with language used for a century to oppose public and low-income housing in white communities.

Demonization of low-income renters here has continued well past the Fair Housing Act of 1968. In the 1970s, Baltimore City proposed that a public housing project called Hollander Ridge be built right up against the Baltimore County border. A community association in Rosedale, a suburb just across the county line from the proposed city housing project, distributed a flyer warning of the “vile social diseases, tuberculosis, juvenile delinquency, and mental ailments” that might accompany the “slum families.” When the city decided to start construction on Hollander Ridge, Rosedale residents demanded a 12-foot brick wall between the project and the county line. The wall never materialized, but in the 1990s, local and federal officials did fund construction of a million-dollar fence.

In 2013, a developer with plans to build 50 homes in Rosedale applied to the state of Maryland for federal low-income housing tax credits, which help developers make rents affordable for low-earning families. By this point, Rosedale’s black population had grown, even reaching a majority in the census tract where the developer wanted to build. Nonetheless, the boundary work continued. “I suspect that everyone knows about Section 8 renters,” the local community association wrote to the developer. “Burglaries, shootings, stabbings, drugs, vandalism, and other crimes go hand-in-hand with Section 8 occupants.”

Rosedale didn’t need a wall or a fence this time around. Maryland had a long-standing policy that allowed county-level elected officials to derail low-income housing tax credit applications with a single vote. In November 2013, Rosedale’s county council representative, a Democrat, submitted a resolution to reject this particular application, and it passed 6–0, with the council’s lone black member abstaining. The developer withdrew the plans. That’s 40 years of turning words into walls in just one town. But Rosedale is not unique. America’s metropolitan regions are full of similar boundaries, whether physical or prescribed in policy.

What’s particularly galling about Trump’s attack on Baltimore is that in some ways, the Baltimore region is a model for the nation in breaking down boundaries. In 2014, the state Legislature removed the local veto. In Thompson v. HUD, a civil rights lawsuit over segregation in public housing, a federal judge found against HUD for failing to consider regionalization as a public housing strategy. The housing mobility program put in place as a remedy has helped more than 4,000 low-income black families move to prosperous white communities, mostly in the suburbs.

Our nation’s current housing crisis might be an opportunity to break down boundaries. Minneapolis and the Oregon Legislature voted this year to remove single-family zoning. In Maryland, the Montgomery County Council voted to ease restrictions on owners who want to build auxiliary dwelling units on their properties. However, many states are passing “preemption” laws to prevent localities from adopting inclusionary housing measures, and even the liberal California Legislature recently shelved a bill to create denser zoning.

Lamont’s work argues for a more inclusive answer to the increasingly urgent question of who gets to live where. Inequality is pushing people apart, leaving us ignorant of the conditions in which people “unlike us” live, Lamont said, and that leads us to lose sight of what we have in common. “It is easier to vilify people if there is a lack of contact,” she said, noting that this decade has seen that lack of contact increasing.

In her most recent research, Lamont found that racial boundaries have become stronger since Trump came to power. When his “every little tweet” is reported, she said, his symbolic boundaries “become taken-for-granted points of reference” for his supporters. “To have someone who has such access to media in terms of diffusing his ideas,” Lamont said, “it has had such a powerful effect on growing racism in the country.” One result of Trump’s rant is that in the public imagination, many will now only see the version of West Baltimore he depicts.

Trump is tapping into a long history of dehumanizing black communities to preserve white advantages. In this case, the advantage he wishes to preserve is electoral. He wants to erect barriers and have voters believe his description of what’s on the other side. He wants us not to notice the problem with imaginary walls: You can see right through them.