"You can argue the exact reverse — that maybe I would have been elected president of the United States," Sanders said at George Washington University to cheers from the crowd.

"The presumption behind that question is that we should anoint candidates for president, that a serious debate or candidates competing against each other is a bad thing for democracy."

Sanders went on to argue that his presidential campaign brought "millions of people into the process" and that the vast majority likely voted for Clinton. And he said that his campaign made Clinton a "stronger candidate" because he pushed her to come out against the Keystone Pipeline and the Trans-Pacific Partnership.

The two former rivals had a tenuous relationship throughout the primary — Sanders and his allies sought to frame Clinton as too cozy with big business, while Clinton and her team cast him as unrealistic and late to the game on civil rights issues.

Sanders did not endorse Clinton for a full month after the primary ended, but he ultimately joined forces with Clinton and campaigned hard for her campaign. When asked about that delay, he said that he wanted to make sure Clinton represented the more than 13 million voters that chose him during the primary.

And he noted that he made 21 speeches in 12 battleground states during the last week of the campaign on her behalf.

"Few people in this country worked harder for Hillary Clinton than I did," he said.