WASHINGTON — For five decades, Bernie Sanders has embraced the label of democratic socialism, one that has defined his political ideology and won him millions of loyal supporters even as it has become a cudgel for opponents seeking to portray him as too radical.
On Wednesday, two weeks before the first set of primary debates, he issued a robust defense of his core political beliefs, delivering a formal address on democratic socialism in what will amount to the most aggressive attempt yet to diffuse voter concerns about his electability.
Mr. Sanders — an independent senator who has not joined the Democratic Party but is running for the Democratic nomination — presented his vision of democratic socialism not as a set of extreme principles but in terms of “economic rights,” invoking the accomplishments of Franklin Delano Roosevelt and the Rev. Martin Luther King Jr. And he argued that his ideology is embodied by longstanding popular programs, including social security and Medicare, that opponents often label as socialist.
Saying that the United States must reject a path of hatred and divisiveness, he said the nation must “instead find the moral conviction to choose a different path, a higher path, a path of compassion justice and love. And that is the path that I call democratic socialism.”
“Today in the 21st century, we must take up the unfinished business of the New Deal and carry it to completion.”
If Mr. Sanders was laying out views that have long shaped his political career, he was also tackling his biggest political vulnerability at a moment when he is falling in some early polls and running second behind Joseph R. Biden Jr. Even before he entered the presidential race, Mr. Sanders has faced skepticism about whether his upend-the-establishment views can appeal to enough voters in a general election, with Republicans — and some of his Democratic presidential opponents — hurling thinly veiled broadsides against socialism.
The issue has taken on outsize importance for a party being pulled to the left by an energized wing of progressives seeking transformational change. President Trump has repeatedly called Mr. Sanders “crazy” and extrapolated the senator’s brand of socialism to all Democrats, seizing on proposals like “Medicare for all” to portray them as far out of the mainstream, and signaling clearly that this will be a major line of attack in the general election.
In his speech, Mr. Sanders was striking back at these negative characterizations.
“Let me be clear, I do understand that I and other progressives will face massive attacks from those who attempt to use the word socialism as a slur,’’ he said, “but I should also tell you that I have faced and overcome these attacks for decades and I am not the only one.”
The president, he said, “believes in corporate socialism for the rich and powerful. I believe in a democreatic socialism that works for the working families of this country.”
Speaking with the use of a teleprompter, Mr. Sanders delivered his speech in a small theater at the George Washington University, a dozen flags behind him. The venue, within walking distance of the bureaus of most major news organizations, was filled with people invited by the campaign, as well as members of the news media, which made up about a third of the audience.
His address appeared similar to one he delivered in November 2015 at Georgetown University during his first presidential bid. In that speech, Mr. Sanders — who at the time was mounting an underdog but surprisingly robust challenge to Hillary Clinton — also presented himself as an heir to the policies and ideals of Mr. Roosevelt and Dr. King and cast democratic socialism as a system that ensures people can have health care, access to higher education and jobs that pay at least a minimum wage.
In the political realm, socialism has become an elastic term that takes on different meanings depending on a person’s viewpoint and ideology. Mr. Sanders has taken pains to draw a contrast between his brand of democratic socialism and the kind that prescribes a command economy and government-owned industry. But at times in his long career in public service, he has also advocated for some policies that leaned toward a more traditional definition of socialism.
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In the 1970s, for example, he argued for nationalizing some industries, including energy companies and banks. And as mayor of Burlington in the 1980s, he went further than many Democrats in supporting socialist leaders. Throughout his political career, he has spoken of revolution, espousing a sympathy for the working class and the poor, whom he argues are suffering at the hands of profit-seeking corporations and the rich and powerful who lead them. One of his political heroes is Eugene V. Debs, the labor organizer.
While he remains popular with many in the progressive left, Mr. Sanders has been working in recent months not only to expand his base but also to retain the voters who supported him in 2016. A recent poll from the Des Moines Register and CNN showed that Mr. Sanders had lost ground over the last three months among likely Iowa caucusgoers, even as Senator Elizabeth Warren and Mayor Pete Buttigieg of South Bend, Ind. have surged.
A poll by Monmouth University released on Wednesday, an hour before Mr. Sanders’s speech was set to begin, showed Ms. Warren had surpassed Mr. Sanders among Democratic voters in Nevada, a key early state, with 19 percent support to Mr. Sanders’s 13 percent.
The rise of Ms. Warren in particular has worried Sanders supporters, who see her as an ideological ally who is nevertheless targeting some of the same voters who were drawn to Mr. Sanders in 2016.
Jen Psaki, a Democratic strategist and former White House communications director for President Barack Obama, said she viewed Mr. Sanders’s speech as a direct effort to take the reins back from Ms. Warren.
“The speech is a pretty clear indication he is feeling the heat from Elizabeth Warren’s recent momentum among progressive voters and recognizes that if he doesn’t do something dramatic she will overtake him,” she said. “It is his attempt to reclaim the anticapitalist mantle he ran on in 2016 — but the problem for Sanders is now there are more progressive options and the same playbook probably won’t work.”
It could also play into Mr. Trump’s hands.
“Most Democrats running don’t subscribe to Bernie Sanders’s democratic socialism and his economic policies,” said Mary Anne Marsh, a Democratic strategist in Boston who worked for John Kerry and Ted Kennedy. “Ultimately, Bernie Sanders giving this speech will appeal to his base and no one else, and it gives fodder to Trump and the Republicans.”
But among his supporters at the speech Wednesday, Mr. Sanders’s philosophy seemed perfectly reasonable.
“In America we embrace a lot of socialist policies already, like public education and parks,’’ said one attendee, Jeremiah Lowery, 33, who works for an environmental organization. “Bernie is just moving America forward by his full embrace of democratic socialism.”
In a telephone interview on Tuesday, Mr. Sanders said that he had planned for some months to deliver this address during this time frame, and that it would elevate human rights “in a way that I think has almost never been raised in a political campaign.”
“It’s going to provoke, I know, a fierce debate,” he said. “I eagerly look forward to President Trump’s tweets.”
He also said he would preview programs he plans to introduce, including on jobs and retirement security.
“This is a debate that the American people have got to have: What are we entitled to as human beings?” he said.