It’s essential that the Democratic Party becomes more of a grassroots organization, Sen. Bernie Sanders (I-Vt.) believes.

That would enable them to become an effective opposition to Donald Trump’s looming presidency following last week’s election defeat to the Republican president-elect, he told “Late Show” host Stephen Colbert Monday.

“The truth is, Democrats should not be losing to a candidate who insults so many people, who wants to give huge tax breaks to the top two tenths of 1 percent and who rejects climate change,” Sanders said.

“How are we losing these elections?” asked Sanders, who lost out on the party’s presidential candidacy to Hillary Clinton. “Something is fundamentally wrong and what I’m trying to do right now is bring about structural changes in the Democratic Party so that it becomes a grassroots party.”

The key solution was to transform itself into a party that “feels the pain of working-class people, of the middle class, of low-income people, of young people.” And the way to make that happen was for people to get “heavily involved in the political process.” “When millions of people stand up and fight back we will not be denied,” he added.

Watch the first segment of the interview above, and the concluding part below: