Veterans of the Obama administration say that the candidates have a lot of work to do to convince voters — even those who reject Mr. Trump’s worldview — to focus on their approaches to building alliances, using force and competing with an aggressive Russia and a rising China.

“Every presidential campaign I’ve ever been a part of, there’s a commander in chief ad,” Wendy Sherman, who conducted the day-to-day negotiations with Iran for the 2015 nuclear agreement, told an audience at the University of New Hampshire last week. “Everybody says at least once, you know, ‘You can rely on me at 3 in the morning.’” But the issues are “rarely central except when we’re in crisis.”

She noted that only a month ago, with the targeted killing of Maj. Gen. Qassim Suleimani, the Iranian military leader, “we were on the brink of war.”

And the Democrats, it turns out, even disagree on whether that killing was legal, or wise.

Differing views on the use of military force

Perhaps the most striking takeaway from the survey was that the candidates have sharply different views on what circumstances justify the use of military force, aside from responding to an attack on the United States or a treaty ally.

Their disagreements were particularly clear on whether they would consider using force to pre-empt an Iranian or North Korean nuclear or missile test — in other words, to prevent a launch that was meant to prove a country’s capability, but not to attack American territory, troops or interests. Presidents Bill Clinton, George W. Bush and Barack Obama each faced that decision and decided not to strike.

Most of the candidates campaigning as moderates said they would consider it: Mr. Biden, Mr. Bloomberg, former Mayor Pete Buttigieg of South Bend, Ind., Senator Michael Bennet of Colorado and former Gov. Deval Patrick of Massachusetts. Interestingly, one of the most liberal candidates, Mr. Sanders, said the same.