Vermont Sen. Bernie Sanders sent his signature feisty message to President-elect Donald Trump today via an overflow crowd of about 2,000 at his sold-out appearance at UC Berkeley’s Zellerbach Auditorium.

“We are not going backwards. We are going forwards,” said Sanders, who lost a hotly contested race for the Democratic nomination to Hillary Clinton and has since come out swinging at Trump’s proposed policies.

To those discouraged at the prospect of a Trump presidency, he said: “The only option that you have is to figure out what’s the most effective way you can fight back … Despair is not an option.”

Sanders, 75, who has garnered a cult-like following among both millennials and older progressive liberals, was at UC Berkeley to promote his new book Our Revolution: A Future to Believe In, which recounts his roller-coaster campaign run.

His appearance was met with a standing ovation, then more cheers when he told the audience “at this great university” that a “delusional” Trump does not have a mandate.

“Donald Trump lost the popular vote by over 2 million votes,” Sanders said. “And even more importantly, on every major issue facing this country, poll after poll shows that the American people want progressive change.”

“As we move into the Trump era, there are areas where there can be compromise, and there are areas that cannot be compromised,” Sanders warned. “No compromise with bigotry. No compromise. No compromise with racism, sexism, with homophobia, xenophobia.”

On the issue of climate change, which Trump has referred to as a hoax, Sanders addressed Trump directly:

“Mr. Trump, you have to start listening to scientists and not just the fossil fuel industry. Climate change is real. It is caused by human activity. It is already causing horrific problems around the world,” Sanders said. “We are the custodians of this, our only planet. We cannot turn our backs on this planet. We are going to take on the fossil fuel industry and transform our energy system.”

The event, sponsored by two independent booksellers, Mrs. Dalloway’s and DIESEL, was broadcast on giant screens facing lower Sproul Plaza for those who missed out on purchasing tickets but still wanted to hear what he had to say.

“I really respect that he doesn’t take money from corporations, lobbyists and stuff, and he just really cares about people,” said UC Berkeley junior MacKenzie Feldman as she watched long lines of people, young and old, gathering for the event. “I was obviously really bummed when he didn’t win the Democratic election, but then obviously, I was really rooting for Hillary. I just really respect him and feel like he’s an awesome person and speaks his mind.”

When the doors to Zellerbach opened at around 1 p.m., crowds jostled, aiming for front row seats. Once seated, they were entertained by musicians on stage playing the fiddle and the banjo.

Among the audience members were Anna Maria Enriquez and her 12-year-old son Lucas. She described herself as a Latin American feminist socialist.

“I need some hope for what’s coming up,” Enriquez said. “Bernie’s core emphasis on economic equality is right, but it shouldn’t be at the expense of identity politics, specifically support for people of color and Muslims, immigrants and LGBTQI people.”

Sanders, whose wife Jane was in the audience, was introduced by UC Berkeley public policy professor Robert Reich, who served as U.S. labor secretary under Bill Clinton. Reich described Sanders as the “embodiment of progressive populism.”

“Sometimes in American politics comes along someone who changes the way a lot of other people think about politics … Even though they may lose, face defeat, ultimately they don’t lose because they’ve changed the terms of the debate,” Reich said.“He made it respectable to talk about getting big money out of politics, tuition-free higher ed, to talk about getting rid of the militarization of police.”

The audience hissed when Reich brought up Trump.

“We are Berkeley. We are respectful, and we are certainly respectful of people who voted for Trump,” Reich responded. “But that doesn’t mean we have to be respectful of what he represents.”

After Sanders’ speech, audience members asked questions. One question came from a 13-year-old who played hooky to come and see Sanders and asked him about his sources of news “in light of all the fake news.” Sanders responded that corporate media needs to talk about more substantive issues, but said he wasn’t optimistic that would happen.

Others asked how he was able to bounce back in the wake of Trump’s unexpected win.

“It is appropriate when you lose to take a day off,” Sanders said to laughter. “But in all seriousness, when you deal with bigotry and the incredible hurt that it does to people. And when you deal with climate change and understand if we don’t act aggressively that the planet that we leave those kids will be a much less happy planet, you don’t have the option of living in remorse or sadness.”