“I don’t even know how to describe it,” said David Alade, the young developer who, with his partner, bought the building in Detroit’s fabled but faded North End.

“Just completely dilapidated.”

Not today. With marble tiles, gleaming wood floors and gobs of fresh paint, the century-old building has been given a makeover. So has Detroit. And so have cities across the nation.

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You would never know it from reading the president’s Twitter feed, which unloaded anew on urban America last weekend with depictions of Baltimore as a “disgusting, rat and rodent infested mess.” But cities are enjoying a golden age, one that has made them safer, more prosperous and more attractive relative to suburban and rural America than at any time in recent decades.

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Instead of bleeding residents, much of urban America is growing. Rather than scaring away young, educated workers, cities have become a magnet for them. Once a turnoff for corporate investment and development, many urban neighborhoods have become the most coveted places to be.

Without a doubt, the sort of blighted neighborhoods Trump has described still exist, often trapping low-income residents in decaying, high-crime zones as the rest of the city gentrifies around them. Some have even gotten worse, falling further behind as more favored areas sprint ahead.

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While uneven, the urban revival has been broad-based, with nearly every sizable American city benefiting. To those who study cities for a living, Trump’s words seem outdated, a relic of a period in the 1980s and early 1990s when urban America appeared to be in terminal decline. The story this century has been decidedly different.

“Cities have turned around,” said Susan Wachter, co-director of the Penn Institute for Urban Research. “And it’s been a rather stunning reversal.”

The reasons are complex and multifaceted — including a dramatic drop in urban crime rates — with causes sometimes difficult to disentangle from effects.

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To Wachter, much of the transformation comes down to a profound structural shift in the American economy from manufacturing — in which wide open spaces for factories and housing were prized, giving suburbs an edge — to information, in which highly educated people flock to places where they can live, work and play in proximity to others like themselves.

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“Knowledge clusters together,” said Wachter, who was a top official for the U.S. Department of Housing and Urban Development during the Clinton administration.

Demographic changes have been critical, as well. The high-

income young workers of today are delaying both marriage and childbirth. Instead of seeking an expansive yard in which the kids can frolic, they want top-notch restaurants, bars and entertainment options. And they want it all within walking distance, because they are less likely to own a car.

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When such high-end options move in, the neighborhood is bound to change, said Victor Couture, an urban economics specialist at the University of California at Berkeley.

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“It’s exactly these amenities that predict a flood of young educated workers,” he said.

As the tax base has grown, municipal services have improved, making cities even more attractive.

The boom of urban living in this century follows a bust toward the end of the last one. From the 1960s to the 1990s, cities “were going downhill pretty consistently,” said Alan Mallach, a senior fellow at the Center for Community Progress. “They were never entirely rodent-infested disasters. But they were declining.”

At the time, sentiments such as the ones expressed by Trump — who has trained his ire on Atlanta, Chicago and New York, among other diverse and largely Democratic places — were not uncommon. Decades later, there is no shortage of neighborhoods where crime is high, services are poor and poverty is rampant, said Mallach, author of “The Divided City.”

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But writing off American cities as a whole has become a much harder case to make.

The urban turnaround began, Mallach said, in “superstar cities” such as New York, Boston and Seattle. It has long since spread, even to places that had once appeared locked in an inexorable descent.

Detroit — onetime cradle of Motown music, the automobile and a thriving black middle class — seemed to solidify its reputation as an emblem of urban blight in 2013, when the city declared bankruptcy. Residents who could decamp to the suburbs did. Those left behind found themselves in a city with sky-high vacancy and crime rates.

Yet six years later, the city is indisputably on the rebound.

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The downtown, once a ghost town, now hums late into the night. The sound of jackhammers fills the air by day as new glass-paneled buildings rise. Barre studios, blow-dry bars and fancy-

coffee cafes line the streets. The wildflower-strewn riverside park — a concrete no-go zone in recent memory — is filled with runners, cyclists and families out for a picnic or a stroll.

“This city was at a dead stop,” said Michael Isabella, a resident since 1983 who manages a popular water ice shop along downtown’s main drag. “Now everything’s happening. Guys in hard hats. Scaffolding everywhere. It’s like what you see in Manhattan.”

Much of that activity, according to urban experts, is due to one man: Dan Gilbert.

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The mortgage and real estate entrepreneur has invested billions in the city, buying up more than 100 distressed skyscrapers and other properties across the downtown, renovating them and keeping them occupied. Gilbert is revered in the city for investing in Detroit when few others would.

“You can’t give him enough credit,” said Patrick Springstead, owner of a long-running bar and restaurant in Corktown, a historic neighborhood adjacent to downtown. “What changed Detroit? He changed it.”

Although Gilbert’s efforts have been focused on downtown, they have inspired smaller investors to look far beyond it.

Those include Alade, who, with his partner Andrew Colom, overhauled the dilapidated apartment building in the North End.

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The pair of 30-something Ivy League graduates moved to Detroit in 2015 — Alade from New York, Colom from Mississippi — to be part of the revival of a city that has long been central to the American economy and culture but that appeared to be in a death spiral.

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“We bought our first house here when Detroit was still in bankruptcy,” Colom said. “People thought we were crazy. They said ‘Detroit is never coming back.’ ”

Since then, operating as Century Partners, the duo has bought up dozens of abandoned homes, some for as little as $500, renovated them and put them back on the market. They focus on sales to first-time home buyers and others who need help getting in on ­Detroit’s revival.

On one recent day, as they drove past a grand old house in an advanced state of decay, Colom asked his partner, “You think you could save that one?”

Thousands of similar houses have been torn down. Thousands more are set to go as part of a citywide strategy that appears to be paying off, with property values rising, crime and unemployment falling and the exodus to the suburbs ending.

The city could soon be growing again, with the major automakers recommitting to Detroit. Ford Motor Co. is pouring $350 million into the city’s long-abandoned central train station to turn it into a tech campus by 2022. The corporate parent of Chrysler, FCA, is building the city’s first new auto assembly plant in a generation, with Detroit residents given first priority for the 6,500 jobs that come with it.

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Key to Detroit’s success in luring the companies, said Mike Duggan, the popular second-term mayor, was selling them on the vitality of cities.

“I told them, ‘You can find a 500-acre cornfield somewhere. But where are you going to find your workers?’” Duggan said in an interview.

Ironically, the millennials who will be the automakers’ engineers and executives don’t want to live in a place where they have to drive to work.

“They want to be in a place that’s walkable,” Duggan said.

Detroit also offers a ready supply of workers to man the assembly lines. On a recent day, hundreds of people gathered in a church community room to hear a city-sponsored presentation on how to apply.

Among them was Cleveland Wilbourn, 38, who came wearing a crisp button-down shirt and sporting a record of performance at a steelmaker that downsized in December, leaving him unemployed.

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Wilbourn lives in the Brightmoor neighborhood, far from the newfound glitz of downtown and where, he said, the president’s depiction of American cities is not far off.

“Of course our cities are rat-

infested. We don’t have any jobs. We don’t have any schools,” he said. “I hope we’re going in the right direction. But the changes have been very small.”

He just wishes Trump would use his power to push for more successes, rather than seeming to glory in urban failures.