Ras Baraka, the current mayor of Newark, was one of those critics. But he has since reconciled with Mr. Booker, and the two have united around a message of progress sparked by Mr. Booker and brought across the finish line by Mr. Baraka (his re-election slogan last year was “Touchdown!”).

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“If you make your bones here, there is no place out there that can stop you in this country,” Mr. Baraka said, referring to Newark. “I usually tell people he got us on the field, and we took the ball and got us in the end zone. But if you can’t get in the game, you don’t get to win the championship. Senator Booker got us in the game.”

Amid the crowd on a surprisingly hot April afternoon, Mr. Booker basked in the city’s renewal, as evidenced by the street vendors lining the park and the long shadows cast by skyscrapers, both venerable ones and new glass towers bearing the logos of Fortune 500 companies.

“When this city took a chance on me as their mayor, the chief executive of this city, I didn’t wait to bring people together,” Mr. Booker said. He continued: “We got people to invest here. We opened new businesses, created thousands of new jobs — and after 60 years of decline, Newark is growing again.”

Mr. Booker began his campaign for president with a video from the streets of the city he still calls home, and his campaign swings feature the near ubiquitous campaign line of “I got my degree at Stanford but my Ph.D. on the streets of Newark.” But he ratcheted up his focus on Newark in the weeks before the rally.

In a recent campaign swing through New Hampshire, Mr. Booker recalled his much-publicized battle with Conan O’Brien, who as host of “The Tonight Show” in 2009 joked that a better health care plan for citizens of Newark “would consist of a bus ticket out of Newark.” Mr. Booker responded by recording a video “banning” Mr. O’Brien from the city’s international airport. The tiff played out for weeks on social media and late-night shows, before Hillary Clinton stepped in to broker a peace deal.

Mr. Booker, who critics sometimes charge chased fame more than his mayoral duties, paints the whole affair as a clever bit of earned publicity for his struggling city.