CNN moderator Jake Tapper devoted a chunk of Thursday’s Republican presidential debate to the subject of violence at Donald Trump’s campaign events — a disturbing trend that most definitely warranted the attention.

But something was missing from the conversation: An incident this week in which Washington Post reporter Ben Terris saw Trump campaign manager Corey Lewandowski roughly grab the arm of Breitbart News reporter Michelle Fields.

Fields has posted a photograph of her bruises. There’s an audio recording from the media scrum that picked up her conversation with Terris in the immediate aftermath.

But Trump and his campaign are denying any of this ever happened. And now Breitbart itself has sought to cast doubt that it was Lewandowski involved. Fields, meanwhile, has filed a police report against Lewandowski, and Terris tells The Post’s Erik Wemple blog that he saw clearly that it was Lewandowski.

The reason this is still being chewed over is largely because there’s no clear video — a key difference between this episode and several others involving protesters and Trump supporters, including one on Wednesday. In that case, smartphone footage from a rally in Fayetteville, N.C., shows a Trump fan delivering a sucker punch to the face of a protester.

It was that incident Tapper cited in a question to Trump: “Do you believe that you've done anything to create a tone where this kind of violence would be encouraged?”

“I truly hope not,” the GOP front-runner replied.

Tapper pointed out that Trump has remarked on several occasions that he wanted to harm protesters or would endorse others doing it — saying specifically that he would like to punch one in the face last month. The host of “The Lead” on CNN also asked Trump’s three rivals to weigh in.

Yet no one mentioned Fields — an oversight she didn't appreciate. (A CNN reporter asked Trump about it after the debate, but it didn’t result in any real news.) During the debate, she retweeted the following message from Republican strategist Ron Christie.

There is a practical answer to Christie’s question: Because Trump refuses to acknowledge that the Fields incident actually occurred, raising it could have sent the discussion off the rails. Trump would have issued a denial instead of engaging in a conversation about violence with a mutually agreed-upon set of facts. There are so many other episodes that Trump can’t deny — because they were caught on tape — that it made sense for Tapper to pick one of those as his example to force the billionaire to address the problem.

Still, Fields would have made a good follow-up by the moderator — especially because Lewandowski is involved in the incident.

Trump sought to distance himself from violent acts during the debate: “It’s not me. It’s usually the municipal government, the police, because I don’t have guards all over these stadiums.”

But he can’t say “it’s not me” when his own campaign manager manhandles a reporter. This might seem to be an insider-y D.C. media story, but it is entirely relevant to the conduct of the Trump’s campaign and the environment he feeds into. It would have been good to get him to answer for it for all to see.