The moment came with a question about last month's attack on the U.S. consulate in Benghazi, Libya, which killed four Americans, including the ambassador. Did the Obama administration refuse a request for extra security from the embassy, and if so, who's responsible for that? Obama's initial answer was evasive. He said that right after the attack, he ordered his national security team to beef up security at embassies across the region, to investigate why the attack happened and to hunt down the killers. He also accepted ultimate responsibility, saying that everyone in his administration worked for him.
Rather than lunge at the gaps in this reply, Romney suddenly turned empathetic.
"These terrible things may happen from time to time," he acknowledged. Many days passed before the White House told us whether the incident had been a planned assault or the culmination of a demonstration, but, he went on, this might have been the result of misunderstanding, not deception.
Having taken a high road and established that he does understand the way the world works, that, yes, sometimes intelligence is imperfect in the aftermath of an international crisis, Romney could have sharpened his attack on Obama's overall policies in the Middle East.
But instead, he took a swipe that was incredibly lame.
"More troubling," Romney said, was that the day after the attack, "the president flew to Las Vegas for a political fundraiser," and then to Colorado for another fundraiser, when he should have been dealing with the crisis.
You could practically hear trumpets blaring "Hail to the Chief" at the vast opening that this left the president. Obama, with a stern face and a barely suppressed tone of outrage, stated that the day after the attack, he stood in the Rose Garden and denounced the assault as "an act of terror." A few days later, he added, he greeted the caskets of the dead, grieved with their families. To say that anybody on his team "would play politics" on an issue like this, Obama concluded, "is offensive - that's not what we do." Romney then dug himself deeper, claiming that Obama took 14 days before he called the attack an act of terror.
Obama replied, "Look at the transcript." Then, in a peak of drama, Candy Crowley, the CNN reporter who moderated the debate, interjected, "He did, in fact, sir," prompting applause from the otherwise-stone-faced audience. In other words, Crowley said, Obama did call it an act of terror.
At a Sept. 12 press conference, Obama did say, clearly speaking about the attack the day before, "No acts of terror will shake the resolve of this great nation."
Romney was right that it took nearly two weeks before the administration released a definitive statement that the attack was premeditated and that it did not spring from a spontaneous demonstration. But he also allowed that sometimes it takes a while for the true nature of such an event to emerge.
In short, Romney failed to make the attack stick. And he gave Obama an opening - a rare and unnecessary one in a town hall forum - to speak up as the commander-in-chief.
Kaplan is Slate's "War Stories" columnist and author of the forthcoming book, "The Insurgents: David Petraeus and the Plot to Change the American Way of War."