Bernie Sanders delivered a stern warning to Donald Trump in Washington, D.C., on Wednesday night, vowing to vigorously oppose any policy that reflected the hateful rhetoric that the Republican president-elect used on the campaign trail.

“Mr. Trump said a whole lot of things during the campaign,” Sanders said in what was billed as a “major speech” at George Washington University. “And we’re going to hold him accountable.”

The Vermont senator, who lost his bid for the Democratic presidential nomination to Hillary Clinton, said he is already holding Trump accountable on at least two issues. The first: Trump’s appointment of former Breitbart CEO Steve Bannon — who has championed white nationalism — as his chief White House strategist.

“This country, since our inception, has struggled to overcome discrimination of all forms: racism, sexism, xenophobia and homophobia,” Sanders said. “Over the years, we have made progress in becoming a less discriminatory and more tolerant society — and we are not going backward. The appointment by President-elect Trump of a racist individual like Mr. Bannon to a position of authority is totally unacceptable. … I call upon Mr. Trump to rescind the appointment of Mr. Bannon.”

The second: Trump’s 2012 assertion that global warming is a hoax invented by the Chinese — which came into full relief when Trump tapped Myron Ebell, a noted climate-change skeptic, to oversee the transition of the Environmental Protection Agency.

“We need to tell Mr. Trump to read up a little bit on science,” Sanders said. “Start listening to the scientific community, and not the CEOs of the fossil fuel industry.” 

“I will keep an open mind to see what ideas Mr. Trump offers and when and how we can work together,” he wrote in a New York Times op-ed published last Friday. “Having lost the nationwide popular vote, however, he would do well to heed the views of progressives. If the president-elect is serious about pursuing policies that improve the lives of working families, I’m going to present some very real opportunities for him to earn my support.”

But in an appearance on “The Late Show With Stephen Colbert” on CBS Monday, Sanders painted a bleak picture when he was asked for his “worst-case scenario.” 

On Wednesday night, the self-described democratic socialist did, however, offer a glimmer of hope, urging fellow progressives to become more politically active.

“We can stop Mr. Trump — or anyone else — from doing bad things,” Sanders said.