Bernie Sanders didn’t sign up for a foreign policy election. Yet that's exactly what he's getting in the wake of the Paris attacks, a shift that's turned the primary contest — temporarily at least — into a test for commander in chief.

For the Vermont senator, it's already proving discomfiting as he faces off against an already-dominating front-runner on terrain that's familiar to her as a former secretary of state.

As Hillary Clinton works to remind Democratic voters of her experience — “I spent a lot of time in the Situation Room as secretary of state and there were many very difficult choices presented to us,” she said on the Des Moines debate stage on Saturday — Sanders suddenly finds himself forced to deviate from the stump speech he’s effectively been honing for decades. His once-soaring campaign, until recently firmly welded to a focus on domestic policy and income inequality, is now reliant on a work-in-progress argument that leans further into his criticism of Clinton’s 2002 vote to authorize the Iraq War.

There's a sense of urgency to Sanders' task of proving he belongs in the national security discussion, said Democratic operative Chris Kofinis, who ran a focus group of 33 undecided Iowa Democratic caucusgoers during the debate. That group unanimously decided that Clinton would be a stronger commander in chief after the forum, and Kofinis found that her message on terrorism resonated particularly strongly with the Iowans.

“It was pretty overwhelmingly clear. Not surprising given her experience as secretary of state, but two weeks ago the issue of terrorism wasn’t at the forefront of the Democratic debate. Now it — at least for the time being — definitely is at the front,” Kofinis explained on Monday. “Sanders had an electability issue going into this debate. I’ve done four groups now in Iowa, and every group says the same thing: We love his message but we’re not sure it’s electable. He needs to address that head on, and that’s now even more difficult because he has to do that as well as address the commander in chief question."

“When these kinds of drastic international events happen, they usually change the dynamic of the race, and you either react to it or you don’t. The perception is that he hasn’t, and he has to,” Kofinis added.

Sanders has yet to schedule — or even write — the military policy speech that his top strategist, Tad Devine, has long urged him to deliver. He has only one public rally scheduled for this week — Monday in Cleveland. It's a reflection of doubts within the campaign, say those familiar with its thinking, that the issue of responding to the attacks will drive voters’ thinking for more than a few days.

Nonetheless, Devine and other top aides expect Sanders to start making his argument shortly.

First, he will more forcefully frame Clinton’s Iraq vote as a question of basic judgment: “Arguably the most important foreign policy decision in the last 30 years," said Devine.

Sanders will also elaborate on his hope to build a new coalition against the Islamic State, continuing a discussion he began — though without offering specifics — at a rally in Indianola, Iowa, on Sunday night.

“We have a different point of view, but Russia has got to join us,” he said the evening after the debate, in the midst of a sprawling speech that focused primarily on the middle-class issues he’s accustomed to addressing. “We are concerned about Iran, but Iran has to join us. We have concerns about Saudi Arabia, but Saudi Arabia has to join us. If all over the world these attacks are taking place, the world has got to come together."

The senator, who often votes against increasing funding for the Defense Department, will argue that the country should shoulder less of the burden in responding to — and paying for — international crises.

If Sanders makes this account central to his appearances in the coming days, it will represent a dramatic departure for a lawmaker whose foreign policy work in the Senate is largely limited to his membership in the Veterans’ Affairs Committee. That background hasn't been a hindrance in a race that mostly centered on economic issues, as evidenced by the recent exits of both of the internationally focused candidates pursuing the Democratic nomination — Jim Webb and Lincoln Chafee.

Instead, the candidates have mostly sparred over Wall Street regulation, environmental policy and immigration. As a result, their Saturday debate opened a new window into their thinking.

Sanders traveled to Des Moines with Senate staffers who’ve been briefing him on foreign policy, but after his opening statement veered away from the Paris attacks and toward the “rigged economy” — a stark contrast to Clinton and Martin O'Malley, who stuck to responding to the attacks — Sanders immediately faced new scrutiny.

Those questions about his preparation to be commander in chief were compounded before long, when Sanders told moderator John Dickerson that he stood by his claim that climate change is the greatest threat to national security — a position far out of sync with the polls.

“Climate change is directly related to the growth of terrorism,” insisted the Vermonter, before his campaign circulated a fact-sheet backing up his claim that the military agrees with his assessment. “And if we do not get our act together and listen to what scientists say, you’re going to see countries all over the world — this is what the CIA says — they’re going to be struggling over limited amounts of water, limited amounts of land to grow their crops, and you’re going to see all kinds of international conflict."

While he recovered in time to engage Clinton and O’Malley in a spirited debate over the country’s role in the Middle East, the exchanges highlighted his difficulties convincing voters of his viability for the job: a pre-debate November CBS/New York Times poll showed that 53 percent of Democratic primary voters were “very confident” in Clinton’s “ability to handle an international crisis,” compared with just 16 percent for Sanders.

Nonetheless, his team remains wedded to the strategy of using the sudden focus on foreign policy as an opportunity to underscore Clinton’s past errors, rather than his own grand plan for the Middle East.

“I don’t think she in any way distinguished herself on foreign policy during the debate. She was secretary of state at the time ISIL emerged, so she was on defense,” said a high-level Sanders aide, previewing a further line of discussion from his candidate. “She does know a lot of names and places and inside facts, but other than that she didn’t distinguish herself."