Watch Bernie Sanders Tell Hillary Clinton to 'Move Forward' on 'Colbert'

"[Clinton] ran against the most unpopular candidate in the history of this country, and she lost," Senator said. "But our job is not to go backwards."

Bernie Sanders responded to Hillary Clinton's harsh criticisms in her new book on 'The Late Show.'

Bernie Sanders responded to Hillary Clinton's criticisms on Thursday's Late Show, telling his former political rival to "[move] forward" instead of focusing on their clashes during the 2016 presidential campaign.

Host Stephen Colbert read a critique from Clinton's upcoming memoir, What Happened, aimed at Sanders: "Your attacks caused lasting damage, making it harder to unify progressives." 

In response, the Vermont Senator straddled the line between throwing shade and letting bygones be bygones. "Secretary Clinton ran against the most unpopular candidate in the history of this country, and she lost," he said. "And she was upset, and I understand that. But our job is not to go backwards – it's to go forwards."

Sanders – whom Stephen Colbert described as a "former Democratic presidential candidate, current independent senator and raspberry seed in Washington's wisdom tooth" – was promoting his own book, Bernie Sanders Guide to Political Revolution. In keeping with the text's theme, he noted that his campaign helped unify young progressives and spread "grassroots activism."

"As a result of our campaign, millions of young people began to vote for the first time, became engaged in the political process," he said. "We're seeing young people all over this country, working class people, running for office – from school board to Congress."

After Colbert asked the Senator for his direct response to Clinton, who is appearing on The Late Show later this month, Sanders continued with his message of political unity, citing pressing issues like climate change, raising minimum wage and universal healthcare. "We have got to stand together again Trump's efforts to divide us up, take on the billionaire class and make that political revolution so that we have a government that works for all of us and not just the one percent," he said.

"We need her help to go forward," he said of Clinton. "Let's not keep arguing about 2016. Let's get together, take on Trump's desire to divide us up. Let's go forward with a progressive agenda. Ask her if she'll do that."

However, Sanders had a tougher time coughing up a compliment for Trump. After a long exhale, a few stammers and a series of "uhhh"s, the Senator cited two of the president's campaign promises – reforming the pharmaceutical industry and investing in infrastructure – on which the politicians see eye-to-eye.