Ever since Hillary Clinton launched a TV ad last week asking voters who they'd want to answer the "red phone" in the event of an international crisis, national security has been back at the center of her contest with Barack Obama.
Obama countered Clinton had already failed such a "red phone" test when she voted to support the Iraq war in 2002. And in a news conference on his campaign plane he questioned her claim that her time and extensive global travels as first lady count as meaningful experience in foreign policy.
"One of the things I hope people start asking is what exactly is this foreign experience she keeps claiming," he said. "I know she talks about visiting 80 countries. It's not clear: Was she negotiating treaties or agreements or was she handling crises during this period of time. My sense is the answer's no."
On her side of the ledge, Clinton offers this: "I don't talk about the conversations that I had with my husband in the White House."
It's a coy reply, citing a certain need for confidentiality. But then she adds: "But obviously I was there for a lot of phone calls at different times of the day and night, and I have a very clear idea of what it takes to be prepared and ready to not only answer the phone, but to then make the decisions that are required depending on what the crisis is."
Each campaign has trotted out high-profile surrogates to press the case on the issue. Clinton appeared with several retired generals endorsing her credentials as commander in chief.
Obama supporters have their doubts, as did former Sen. Bill Bradley on PBS's NewsHour.
"I mean, the role of a first lady is very important, I don't doubt that and I think in certain diplomatic situations she was helpful," he said, "but in terms of being a person ready to go with the so-called red phone at the bedside on Day One is just incorrect given what the whole facts are."
The debate has not just been between the Democrats.
"Can I just say, please keep running those 3 a.m. phone call ads about who you want to answer the phone because we, um, we like those," McCain's foreign policy adviser Randy Scheunemann said at an event sponsored by the Council on Foreign Relations on Friday.
McCain cites his military experience, time as a POW in Vietnam, and 21 years as a U.S. senator.
"I'm certainly the most experienced and qualified to answer that phone," he said.
But for voters trying to sort this out, there's no obvious answer, says Robert Dallek, biographer of presidents such as Franklin D. Roosevelt, John F. Kennedy, Lyndon B. Johnson and Ronald Reagan.
For starters, Dallek says, history shows it's impossible to know how a prospective president will react to crisis before taking office.
"What are the conditions that they're facing? What exactly is that moment in time like? Nobody can know," he says. "Nobody can predict with any certainty."
He points to Kennedy's military experience and extensive world travel, which didn't prevent the Bay of Pigs blunder. And there's the example of Lyndon Johnson and Vietnam. Dallek says history will judge how the next president responds to his or her first crisis.
Voters, however, have to make their decision first.