Sanders, who, like Clinton, is seeking the 2016 Democratic nomination, spoke with reporters at a stakeout normally reserved for leadership from both parties. He used the impromptu press conference to underscore differences the two have on a wide range of issues, including trade, the war on Iraq, climate change and national security.

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Pointing to his vote against the North American Free Trade Agreement (NAFTA), Sanders said that he believes the trade deal has "been disastrous for American workers. ...Secretary of State Clinton I believe has a different view on that issue."

He also cited his vote against authorizing the war against Iraq. Clinton, who was a senator at the time, has faced criticism for her vote supporting the war, and she told reporters earlier this year that she "made a mistake."

On climate change, he touted his opposition to the Keystone pipeline adding that "I think Secretary Clinton has not been clear on her views on that issue."

Tuesday's lunch offered a rare opportunity for Sanders and Clinton to meet face-to-face since they launched their presidential bids, where Sanders has gained ground on Clinton in key states including New Hampshire.

Asked about the policy lunch, Sanders said that "I've known the secretary for 25 years. ... I like her. I respect her."