Sanders spent more than an hour at Georgetown University, largely explaining that his definition of "democratic socialism" is identical to the same fiery populist points he's been making throughout his campaign. But he also wanted people to know that is very much not "socialism."
Sanders said the inequality in the United States mirrors the same gap between wealthy and poor that Roosevelt fought during the Great Depression and that Roosevelt faced the same claims that his programs were "socialist."
"In other words, real freedom must include economic security," Sanders said. "That was Roosevelt's vision 70 years ago. It is my vision today. It is a vision that we have not yet achieved. And It is time that we did."
The speech, which amounted to a more precise explanation of the same points he has made in his upstart presidential bid, was a key moment for the Vermont senator. It had long been promised, but the campaign pushed it back a few times, leading to some speculation that Sanders might can the idea altogether. After a bump in the polls in late summer, Sanders has recently fallen behind Hillary Clinton.
Jeff Weaver, Sanders' campaign manager, said that the Vermont senator worked "intensely" on this speech over the last few days and took personal ownership of it.
The crowd of mostly Georgetown students, packed into the university's Gaston Hall, cheered loudly throughout his lengthy speech, which also touched on how he would address ISIS
and a broader foreign policy.
At one point Sanders brought up King's assessment that "'This country has socialism for the rich, and rugged individualism for the poor'" as part of a broader strategy aides said was to show his ideas are hardly foreign.
But he was also defensive at times.
"So the next time you hear me attacked as a socialist, like tomorrow, remember this: I don't believe government should take over the grocery store down the street or own the means of production, but I do believe that the middle class and the working families who produce the wealth of this country deserve a decent standard of living and that their incomes should go up, not down," Sanders said.
Weaver called it a "very important speech," but would not say it was the most important speech of the campaign. Sanders' goal was to demonstrate "that his agenda is firmly in the mainstream of the Democratic Party prior to the party leadership's movement to the right in the 1990s."
"He really represents the Democratic Party regaining its connection with its roots," Weaver said. "The Democratic electorate has already moved there and Bernie is pulling the institutional party to where the Democratic rank-and-file already is."
'I am not a pacifist'
Sanders expanded on his foreign policy ideas at the end of the speech -- building out points he began making after the Paris attacks but had yet to detail until Thursday.
He criticized U.S. allies in the Middle East for not spending more to fight ISIS and specifically cited reports that Qatar would spend more than $200 billion preparing for the 2022 World Cup.
"Wealthy and powerful Muslim nations in the region can no longer sit on the sidelines and expect the United States, our young men and women, our taxpayers to do it for them," he said.
Sanders touted his votes against both the Iraq War and the Gulf War
, but he also cautioned that he's not a pacifist and cited his support for action in Kosovo and Afghanistan.
"No, I am not a pacifist. I think that war should be the last resort," he said.
Will 'democratic socialism' sell?
Sanders has long embraced the socialist tag, and used his speech to link his values to Roosevelt's 1944 call for a "Second Bill of Rights" with promises of well-paying jobs, housing, health care and general economic security for Americans.
Using "socialism" to describe anything in American politics has long been a non-starter, especially for Democrats. Donald Trump
recently got in a dig on Sanders, calling him a "communist" more than a "socialist."
Attitudes among voters have softened, but a Gallup survey this past June
found that a "socialist" would still have trouble getting elected with that label -- with 47% saying they would consider voting for a "socialist," but 50% of voters polled ruling out the idea altogether.
For comparison, 58% of voters polled said they would consider an atheist for president, 60% would consider a Muslim candidate, 73% would consider an evangelical Christian and 74% were open to a gay or lesbian candidate.
Sanders speech comes just hours after Clinton gave a speech at the Council for Foreign Relations in New York that outlined her plans to combat ISIS.