But Sanders never agreed -- until now.
Sanders is preparing to give a major speech soon to define and explain what it means to be a democratic socialist. This is a speech, advisers tell CNN, he committed to only very recently, an acknowledgment that the stigma around the term could be holding him back from truly challenging Hillary Clinton for the Democratic nomination.
"He needs to reassure voters, especially in Iowa and New Hampshire, that they will be nominating someone who does have a real chance of winning," Sanders strategist Tad Devine told CNN. "The idea that this word is going to get in the way of voting for him, we're going to expose it for what it is. That this discussion over the term will not stop him as a candidate."
The senator started writing the speech Tuesday in Washington, Devine said, with the plan that he will deliver it before the next Democratic debate on November 14.
Sanders will talk about what his political philosophy means in broader terms and what it has meant in practice as mayor of Burlington, Vermont, and in Congress, he added.
He'll talk about how his political philosophy is seen through the eyes of others but will contrast that with the "American perspective" to show how programs such as Social Security and Medicare have made a difference in people's lives over the years and were once attacked as socialism, an argument he has started to make in small ways already on the campaign trail.
"When you go to your public library, when you call your fire department or the police department, what do you think you're calling?" Sanders said Sunday in Iowa. "These are socialist institutions."
But in what seems to be the heart and main goal of idea of the speech, to reassure voters, Sanders will attempt to address the question about him head on: How a socialist can win the White House.
"He will reassure people he is running to be the President of the United States, on the platform of the Democratic Party, and that he will work within that system, as he has for a long time, but also will bring his own philosophy," Devine said. "He would not be the nominee of a Democratic Socialist Party, he wants to be the nominee of the Democratic Party."
A new poll out Thursday of New Hampshire Democrats by WBUR showed that only 48% said they would accept a socialist president. Some 36% said it would be flat-out unacceptable.
But "democratic socialist" polls better. Some 62% of those surveyed said a democratic socialist president would be acceptable, 20% of respondents said it would not be acceptable and 17% said they didn't know.