Bernie Sanders isn’t one for ducking questions. So when Stephen Colbert asked the Vermont senator what he thought about Hillary Clinton’s new book on Thursday night’s The Late Show, Sanders had a lot to say.

“Look, Secretary Clinton ran against the most unpopular candidate in the history of this country and she lost and was upset about that and I understand that,” Sanders told Colbert. “But our job now is really not to go backwards. It is to go forward. It is to create the kind of nation we know we can become. We have enormous problems facing us and I think it’s a little bit silly to keep talking about 2016.”

In her new book/election postmortem What Happened, Clinton dissects the many factors that contributed to her stunning, once unthinkable election loss. According to her, Sanders’ relentless attacks on her policies “caused lasting damage,” and were instrumental in dividing progressives as the general election loomed. “President Obama urged me to grit my teeth and lay off Bernie as much as I could. I felt like I was in a straitjacket,” she writes.

When Colbert specifically asked Sanders what he thought about Clinton’s accusation that he “damaged America by damaging Hillary Clinton,” Sanders didn’t mince words.

“Actually,” Sanders said, “the case is the progressive movement today and grassroots activism is stronger than it has been in many, many years.” Sanders seemed to suggest that Clinton is still licking her wounds. His belief that re-litigating the past could hurt the Democrats moving forward is a popular feeling among members of the party. And with Clinton’s book tour set to begin on September 18, some democratic lawmakers are worried that dwelling on the past could damage the party’s chances of winning a majority in the upcoming midterm elections.

“There is a collective groan whenever there’s another news cycle about this,” California Democratic representative Jared Huffman told Politico recently. “I’ve always been a looking-forward kind of a guy,” added Democratic senator Ron Wyden. “I think I’ll leave it at that.”

But with Clinton set to appear on Colbert on September 19, it doesn’t sound like any of us will be moving on any time soon.