That plan has now changed.
Tad Devine, Sanders' top strategist, told CNN on Wednesday that the speech will now come after the debate in Des Moines, citing the senator's busy schedule.
But while Sanders has started writing the speech, aides have said it has been difficult to get him to agree to give it.
Devine maintained Wednesday that the senator is "not the least bit apprehensive" to give the speech.
"He's looking forward to it, but these speeches take some time," he added.
In the lead-up to his campaign, Sanders was routinely asked about his "socialist" views. He would usually rebut the questions with a quick disclaimer: He is a "democratic socialist," not a socialist.
And lately he has suggested there are aspects of socialism already present in American society.
"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 last month in Iowa. "These are socialist institutions."
At the first Democratic debate, CNN asked Sanders if he considered himself a "capitalist."
"Do I consider myself part of the casino capitalist process, by which so few have so much and so many have so little by which Wall Street's greed and recklessness wrecked this economy? No, I don't," he said forcefully. "I believe in a society where all people do well. Not just a handful of billionaires."
There is a difference between socialism and democratic socialism. While socialists believe in government takeover of certain large sectors of the economy, democratic socialists see government intervention -- along with private businesses -- in certain portions of the economy, like college and health care.
To date, though, Sanders has not said that outright in a major forum.
"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 last month. "He would not be the nominee of a Democratic Socialist Party, he wants to be the nominee of the Democratic Party."
Sanders' Democratic rivals have subtly knocked the senator on this issue, too.
"I am a proud Democrat," Clinton said in Iowa in September. "I am a true Democrat."