When asked why he has yet to endorse Clinton, Sanders responded, "Because I have not heard her say the things that I think needs to be said."
Throughout the primary process, Sanders and Clinton have engaged in intense battles over what it means to be a progressive. Clinton has opted for pragmatism and incrementalism, while Sanders has taken an ambitious, mass politics approach, one that brings new voters into the political process and fights for significant changes to America's political and economic systems.
"What we are trying to do is make certain that she is going to come out very strongly in moving toward making public colleges and universities tuition free," Sanders told Colbert, referring to one of the key components of his platform.
Until Clinton becomes more open to the changes Sanders has sought throughout his political career, it seems likely that Sanders will continue to withhold the formal endorsement.
However, Sanders also acknowledged that he has been in conversation with the Clinton campaign, insisting that he and his supporters will continue to fight for an America that works for everyone, not just the wealthy few, indicating that he will be willing to cooperate with Clinton in pursuit of this goal — provided that his supporters are given a voice throughout the process.
"We got 13 million votes," Sanders said, "we got in virtually every primary and caucus the vast majority of young people — people 45 years of age or younger, and what those voters are saying to the establishment, to Secretary Clinton — 'Hey are you gonna stand up for us? Are you gonna raise the minimum wage in fact to 15 bucks an hour?'"
Disagreements between Bernie Sanders and Hillary Clinton run deep, but Sanders has also expressed the necessity of stopping Donald Trump, indicating that, while he and Clinton do not occupy common ground on many issues, they both want Donald Trump to be stopped.
Expressing a fear that has become common among Democrats, Colbert asked Sanders what he would say to those of his supporters who are considering voting for Trump.
The Vermont senator's response was straightforward.
"My supporters," he said, "are smart enough to know that we do not want a bigot to become president of the United States."