“In debates, what really stood out to me is just how different he sounds,” said Philippe Reines, who played Mr. Trump in mock debate sessions with Hillary Clinton in 2016. “He has the vocabulary of a fifth grader. But to some people, that is normal talk. If you’re sick of political speak, normal talk is going to be appealing.”

Democrats who specialize in presidential debate prep said Mr. Trump succeeded because his shock-jock personality was well suited to the current media environment. When debates are covered in real time and voters are taking them in with play-by-play commentary on Twitter, the behavior of candidates onstage is dictated by the need for short, punchy moments that differentiate them from the pack. Mr. Trump did not create that dynamic, they said; he just benefited from it.

“Since the rise of contemporaneous coverage, there is a greater imperative to have a breakout moment in the very early minutes of the debate,” said Karen Dunn, a lawyer who helped oversee Mrs. Clinton’s debate prep and is currently unaffiliated with any campaign.

But the ways that Mr. Trump was able to appeal to Republican primary voters do not necessarily translate to the other side of the aisle — or to anyone else.

“Trump has a superpower, which is utter shamelessness that is hard for sane humans to replicate,” said Tim Miller, who served as a campaign adviser to former Gov. Jeb Bush of Florida, one of Mr. Trump’s opponents. “I think he has probably opened up a wider prism of possibilities in how a candidate could act in debates successfully. But I don’t know that we’ll see a lot of traditional politicians go that route.”

The field of Democratic candidates may be diverse in terms of race, gender and age. But what has emerged in the top tier is a group of conventional presidential candidates, all current or former elected officials. That lineup bucked a brief moment of expectation that someone — Mark Cuban, the billionaire; Michael Avenatti, the celebrity lawyer — would emerge as a Democratic version of Mr. Trump.

Democratic strategists said that was a good thing for the party.

“You don’t beat him by matching him. You beat him by appealing to some of his voters,” said Jennifer Palmieri, who served as communications director on Mrs. Clinton’s 2016 campaign. “And you beat him because we’re able to turn out a lot of Democrats.”