Korb was among about a half-dozen foreign policy experts who spoke to POLITICO on Friday after Sanders’ campaign cited them as recent sources of advice for the Vermont senator. At least half of them say they have only spoken to Sanders once or twice in the past year.

During an appearance on CNN’s “State of the Union” Sunday, Sanders declined to offer specifics when pressed by the show’s host, Jake Tapper, to name any generals or former defense officials “on whose counsel you rely.”

“Look, we have been talking in the last month to, you know, many, many, many people who are very knowledgeable about national security issues and foreign policy issues. And I am confident that we have the judgment and the experience to do what has to be done for the American people,” Sanders replied.

When Tapper continued to press for specific names, Sanders first said “there are too many,” before offering: “Larry Korb is one.” He did not name any others.

In the Senate and on the campaign trail, Sanders spends far more time discussing economic issues like income inequality than military and diplomatic ones. The campaign of his Democratic primary rival, Hillary Clinton, argues that Sanders is out of his depth on security issues. Clinton has given multiple speeches on foreign policy and national security, while Sanders has decided not to follow through on a commitment to give his first foreign policy address before Monday’s Iowa vote.

Whether Clinton’s attack is resonating with domestic-minded Democratic primary voters is unclear. But questions about Sanders’ foreign policy views and credentials will likely grow if he wins early primary contests against Clinton and gains momentum toward his party’s nomination.

Korb told POLITICO on Friday that he met with Sanders at the senator’s invitation, “just like I did with Rand Paul” and others. “He was very attentive, was taking notes, asks good questions and is very thoughtful about the issues,” Korb added, saying the men discussed topics that included the Middle East, climate change, nuclear modernization and the defense budget.

Korb is a former assistant secretary of defense under Ronald Reagan. But his current employer, the Center for American Progress, was founded and is largely staffed by allies of Hillary Clinton. “I am not involved in [Sanders’] campaign or anybody’s,” Korb said.

Korb noted that he also spoke with Sanders about the Iraq War soon after Sanders was elected to the Senate in 2006.

Another Democratic foreign policy expert who has spoken with Sanders in recent months described the Vermont socialist as “intelligent and informed but not deeply immersed in the subject matter” and still “trying to formulate his positions.”

The ambiguity about Sanders’ foreign policy team is a stark contrast to Clinton’s campaign, which maintains several foreign policy working groups manned by hundreds of experts and former government officials. The groups are coordinated by Laura Rosenberger, a former top aide to deputy Secretary of State Antony Blinken who manages policies, messaging and strategy on the national security issues for the campaign. One of Clinton’s closest confidants is Jake Sullivan, a former top State Department official during her tenure. Sanders’ campaign has yet to publicly identify a full-time foreign policy staffer.

The lack of foreign policy emphasis is also a departure from the last Democratic insurgent campaign. By the time of the 2008 Iowa caucuses, Barack Obama was advised by a slew of well-known Democratic foreign policy hands, including Bill Clinton’s former national security adviser, Anthony Lake; the human rights academic and author Samantha Power; former Bill Clinton State Department officials Gregory Craig and Susan Rice; former Air Force generals Merrill McPeak and Scott Gration; and former Navy Secretary Richard Danzig.

Sanders noted on Sunday, as he typically does when asked about national security, that he opposed the 2003 Iraq War, which he told Tapper is “the most important foreign policy issue in our lifetimes, or at least in the last 20, 30 years.”

Clinton voted to authorize the use of U.S. force in Iraq. She also backed other American interventions — including the 2011 air campaign in Libya — that Sanders calls wrongheaded. But Sanders has supported some recent U.S. military actions, including the 2001 invasion of Afghanistan and Bill Clinton’s air campaign in Kosovo. Sanders opposed the 1991 Gulf War.

“I am confident that in terms of dealing with the Middle East crisis, the need to put together a coalition to prevent our young men and women in the military from getting involved in perpetual warfare, I am absolutely confident that I can handle that issue,” Sanders told CNN.