Indeed, his post was strikingly at odds with his customary posture as all-but-invisible observer.

“On the blog I use very few words and no photos of myself,” said Mr. Stanton, who draws his income from book sales and speaking engagements, and says he has turned down millions of dollars in online advertising. “I don’t want to rent out the influence of ‘Humans of New York’ to any third party.”

Still, he’s not a complete stranger to the camera. Viewers may have caught his appearance on CNN last December as a passionate advocate for the refugees of Syria and Iraq, and one in particular, identified only as Aya. Mr. Stanton wept openly in discussing her plight with the host, Fareed Zakaria.

At Starbucks, he projected a similar intensity, reddening from time to time, describing his work with a missionary zeal. He grasped this reporter’s arm repeatedly to hammer home a point, and from time to time clamped a hand over hers, lest she somehow misinterpret him.

“I think there was a power in speaking directly to Donald Trump, rather than writing a think piece about him, ” Mr. Stanton said. “It also gave people who I think feel very strongly about him a chance to speak directly to him.”

His decision arrived by degrees. In the half-dozen years since he left his job as a financial trader in Chicago (not quite voluntarily, as he has made clear in past interviews) to take up his camera on the streets of Manhattan, Mr. Stanton has snapped and interviewed some 10,000 randomly selected strangers, posting their photos and capsule life narratives on his blog.

He is welcomed at times but just as often rebuffed.

“It’s impossible to do this job without a strong sense of humility,” he said. “That’s what it takes to be able to intensely listen as if each person’s viewpoint was as valid as your own.”