In 2011, Sanders gave an interview to The Guardian. You can read the whole thing here, if you're interested, but I mention all this because one passage stands out --
But, he explains, he is not interested in the White House. '" would likely end up causing a right-wing extremist to be president of the United States. That is not something I would be happy to do," he said.
Then he added for emphasis: "It would likely be a futile and losing campaign. That would not be too smart.'
Clearly he feels differently today, and that's fine. We're all entitled to change our mind. But I do think it would be appropriate for him to at least be asked about this quote, perhaps even during a debate, and let him explain what did change in his thinking.