Aides to Ben Jealous prepped him for days, repeatedly telling him the key words that he should emphasize in the 90-second introduction of his first debate of the Maryland gubernatorial primary.

And then it happened, about halfway through: Jealous was living one of his worst nightmares. He began to stutter. On stage. In front of his rivals for the Democratic nomination.

“I’m running to make sure that we fully fund our schools, that we end killings of unarmed civilians by the p-p-police,” Jealous said. “And make sure that all of us have health care that we can actually afford to use. . . . I specialize in getting big things done.”

The former NAACP chief, who gave a speech at the 2016 Democratic National Convention and is a regular commentator on cable news, is one of 3 million people in the United States who have a stutter, a neurodevelopmental disorder that first exhibits itself in early childhood.

There are genetic factors: More than half of those who stutter have a family member who also stutters. The condition affects more men than women — famous people who stutter include former vice president Joe Biden and actors James Earl Jones and Bruce Willis — and cuts across every culture and economic background.

“People don’t understand the origins of stuttering and don’t know what is underneath the iceberg,” said Vivian Sisskin, a clinical professor in hearing and speech sciences at the University of Maryland who has used a clip of Jealous’s speaking on MSNBC to teach her introductory class on speech pa­thol­ogy.

She said stuttering is not an anxiety disorder, although people often think it is. And when stutterers pause to try to avoid stammering, people sometimes conclude that they don’t know what they are talking about. “When you are trying to hide the stutter, the struggle increases,” Sisskin said.

In August, Jealous visited the National Therapy Center, a small office in Bethesda, and talked with students, teachers and a parent about what it’s like to have a stutter.

“This is one of the big challenges that I had to take on early, just learning how to express myself and my ideas,” Jealous said at the start of the discussion, which was streamed live on his Facebook page.

Most of the time, the Democrat’s speech disorder is not obvious while he is campaigning. But since he announced his bid to unseat Gov. Larry Hogan (R) more than a year ago, Jealous has occasionally experienced abnormal silences — also known as “blocks” — during television appearances, tripped over words during interviews and gotten stuck trying to form a word while sitting with voters in their living rooms or at their kitchen tables.

The Hogan campaign appeared to take advantage of one of Jealous’s “blocks” in July, releasing a video titled “Ben Jealous’ 7 Seconds of Silence” that showed Jealous pausing before answering a question about the Eastern Shore. The video suggested that the pause showed Jealous didn’t know much about the region.

Hogan campaign spokesman Doug Mayer declined to comment on the use of the video. Jealous, a Rhodes Scholar and venture capitalist, said the video was “no different than anything that I’ve dealt with my entire life. I didn’t sweat it.”

Mileah Kromer, an associate professor of political science at Goucher College, said Jealous’s decision to talk openly about his stuttering and his posting about it on social media may appeal to voters.

“All politicians need a moment that humanizes them” Kromer said. “Any time a candidate can show a struggle, one that people can identify with, it can only help. There really is no downside.”

Jealous said when he feels he is likely to stumble over a word that he plans to use, he will replace that word with one, two or three other words to avoid stuttering.

“I think the least understood thing about stutterers is the extra effort we have to put into ... getting our words out clearly,” Jealous said in an interview, again pausing to navigate around a stammer. “When you talk, you know what sounds are likely to catch you up, and you can feel it coming.”

Gerald Maguire, a psychiatrist and stutterer who chairs the National Stuttering Association, recalled using the “word-replacement” technique as a child, when he made a school presentation about his family. He would trip over the word “brother,” so he told the class that he had four siblings: three boys and a girl.

About 5 percent of all children stutter in early childhood, according to the foundation, but most lose the stutter as they get older.

Biden told the American Institute for Stuttering two years ago that he would stand in front of a mirror as a child and recite poems by Yeats and Emerson to try to overcome his stutter.

He said he had declined an invitation to speak at a similar dinner when he was first elected to the Senate, at age 29. “I was afraid if people knew I stuttered they would have thought something was wrong with me,” he said at the gala. “. . .I, too, thought I was handicapped.”

Jealous, who grew up in California in the 1970s, said his parents didn’t know what to do about his stuttering. They were “just patient and just encouraging not to let me — not to let it — get in the way of anything.”

Jealous spent summers with his grandparents in West Baltimore, and recalled his stutter being “perennial entertainment” in their home. There was a bar in their basement where his grandfather would entertain friends. Those friends, in turn, helped a young Ben Jealous cope with his speech impediment.

Robert Watts, a civil rights lawyer who later became a judge, would challenge him to debates, and Jealous would practice how to take him on without stuttering.

“It forced me to slow down,” Jealous said. He eventually learned that he did best when he spoke extemporaneously and with a preacher’s cadence (His mother told him that he didn’t stutter when he sang).

Jealous opened up about his stutter in a 2015 book he co-wrote titled “Reach: 40 Black Men Speak on Living, Leading and Succeeding.” But he has spoken publicly about it only in the last year.

“The biggest impetus to talk about it is to just make it obvious to those young people who are oftentimes struggling in isolation,” he said. “I just want to inspire young people, the way my parents inspired me, to simply not let it get in the way.”