Only one candidate on the stage last night at Houston deigned to mention the Constitution. Never mind that it is the parchment to which the winner of the presidential election is going to have to be sworn. The fact is that it was referenced by only Vice President Biden. That was when one of the moderators, ABC’s David Muir, brought up guns. The exchange that followed was a doozy.

Mr. Muir noted that the audience included survivors of the El Paso shooting. He also noted Sandy Hook, where 20 pupils and six others were slain at school. He noted that there’d been a “groundswell” to get something done and that President Obama assigned Mr. Biden to lead the push for gun control. “If you couldn’t get it done after Sandy Hook,” Mr. Muir asked, “why should voters give you another chance?”

“Because I got it done before,” Mr. Biden stammered. He claimed to be “the only one up here that’s ever beat the NRA,” which turned out to be a reference to the Brady Bill that President Reagan signed. He also argued that after Sandy Hook, gun control became a movement. He suggested that was a “sea change.” He praised Beto O’Rourke for his impassioned remarks after the slayings at Walmart.

Mr. Muir then put the question to Senator Harris and her vow to use executive action against assault weapons within her first 100 days. He quoted Mr. Biden as saying there’s “no constitutional authority to issue that executive order” and “you can’t do it by executive order any more than Trump can do things when he says he can do it by executive order.” Asked Mr. Muir: “Does the vice president have a point there?

Mr. Biden jumped in to say: “Some things you can. Many things you can’t.”

Mr. Muir said he wanted to hear from Ms. Harris.

“Well,” the Coast Democrat said, “I mean, I would just say, hey, Joe, instead of saying, ‘no, we can’t,’ let’s say, ‘yes, we can.’”

That brought laughter from the audience.

“Let’s be constitutional,” Mr. Biden bristled. “We’ve got a Constitution.”

“And yes, we can,” Ms. Harris retorted, “because I’ll tell you something. The way that I think about this is, I’ve seen more autopsy photographs than I care to tell you. I have attended more police officer funerals than I care to tell you. I have hugged more mothers of homicide victims than I care to tell you. And the idea that we would wait for this Congress, which has just done nothing, to act, is just . . .”

The ex-San Francisco DA continued in that vein until Mr. Muir thanked her and turned back to Mr. Biden and asked, “Do you still stand by what you said on an executive order?”

“No,” Mr. Biden replied. “What I said was — the question — speak to constitutional scholars. If in fact we could say, ‘By the way, you can’t own the following weapons, period; they cannot be sold anymore’ — check with constitutional scholars.” And that was the last reference to the Constitution in the entire Democratic Debate. No other candidate or moderator made any further reference to it.

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Correction: Houston was the site of the Democratic debate Thursday evening; the city was incorrectly given in the early edition of this editorial.