Sen. Bernie Sanders (I., Vt.) took a shot at Hillary Clinton for losing to Donald Trump on Thursday night, saying he understood she was upset over losing to "the most unpopular candidate in the history of this country."

The former Democratic presidential candidate appeared on "The Late Show With Stephen Colbert," where he discussed the 2016 election and Clinton's new book What Happened, which delves into her devastating defeat at the hands of Trump.

Colbert questioned Sanders about Clinton's claim that he caused "lasting damage" to her campaign during the primaries through his attacks against her.

"Look, Secretary Clinton ran against the most unpopular candidate in the history of this country, and she lost," Sanders said. "And she was upset about it. Now, I understand that."

Although Sanders explained that he understood Clinton's devastation to losing to what he believes should have been an easy opponent, he says that it is "silly" to be focusing on the 2016 presidential election.

"Our job now is really not to go backwards, it is to go forwards," Sanders said. "And I think it's a little bit silly to be keeping talking about 2016. We got too many problems."

Sanders mocked Clinton Thursday on MSNBC, laughing at the notion that he took any of her ideas.