SPRINGFIELD, Ohio—"If we only had 5,000, I wouldn’t be talking to you right now," says Reverend Barron—real name Kenneth Selig—in what’s become a common refrain in the embattled city of Springfield, Ohio. Barron, a street preacher, food truck owner, and former warehouse manager, has emerged as a colorful champion for the small city as it struggles through an unprecedented transformation. As many as 20,000 Haitian immigrants, city officials say, have arrived in Springfield since the pandemic, swelling its population to almost 80,000. Times were already tough before the rumors started swirling about migrants eating cats, dogs, and geese. "Look man,...