So the senator from Vermont — a state where the largest city has but one black barbershop — has begun trying to make inroads across the South and beyond and the country with black voters, who are perhaps the most crucial pillar in a multicandidate Democratic primary.
Earlier this year, Mr. Sanders invited Representative Cedric Richmond of Louisiana to dinner at an Italian restaurant in Washington, telling Mr. Richmond, the head of the Congressional Black Caucus, that he wanted to work more closely with the group. He recently convened a meeting in his office with two black economists who have researched issues of racial and class inequality. And later this month he is expected to join the Rev. Dr. William J. Barber II, a North Carolina-based black pastor who has risen to prominence as a social justice activist, for a joint event at Duke University.
Yet even as he moves to forge new relationships among African-American leaders and Democrats, Mr. Sanders is demonstrating why it may prove difficult for him to command broad support with a bloc of voters who usually do not rally to the more liberal candidates in Democratic primaries.
Appearing with Mr. Lumumba, the Jackson mayor, at the forum on economic justice, Mr. Sanders was asked how he would engage millennial voters and remake the Democratic Party.
He immediately won applause by declaring that the party’s business model had “failed” and then recalled, as he and many Democrats often do, that the party had lost about 1,000 state legislative seats in the last decade.
But Mr. Sanders also said that these setbacks happened on the watch of “a charismatic individual named Barack Obama,” whom Mr. Sanders also called “an extraordinary candidate, brilliant guy.”
Few in the audience responded adversely, many of them having witnessed firsthand the decline of the state and local party. But the fact that his only mention of Mr. Obama was in reference to Democratic defeats, particularly during an event honoring Dr. King in a heavily black Deep South capital with a painful racial history, struck some critics as tone-deaf and even insensitive.
On Thursday, Mr. Sanders and his top aides responded angrily to the suggestion he had diminished Mr. Obama. The senator tweeted that “some have so degraded our discourse that my recognition of the historical significance of the Obama presidency is attacked.”
The episode was also a reminder of another hurdle in his way: the feud between many Sanders supporters and Democratic leaders and Hillary Clinton loyalists, which has been raging ever since he challenged Mrs. Clinton for the nomination. Mr. Sanders remains very much an insurgent in a party he still has not formally claimed as his own, a fact he made clear in a less remarked-upon part of the same answer: “The establishment,” he said, “doesn’t go quietly into the twilight.”
Mr. Richmond, the Congressional Black Caucus leader, said he did not think Mr. Sanders had slighted Mr. Obama. The mistake Mr. Sanders made, according to Mr. Richmond, was that he did not go the next step and explain why Democrats incurred so many down-ticket defeats during the Obama years.
“The real question is why it happened and it’s no secret: Everybody underestimated the backlash that would come to the first African-American president,” he said.
As Mr. Sanders seeks to gain support from black voters, the Jackson forum was also notable for what the senator did not say to the audience, which skewed young and was almost evenly divided between blacks and whites.
While briefly noting that Dr. King had been a “major political inspiration” for him, Mr. Sanders said nothing about his history as a civil rights activist and his arrest demonstrating against segregation as a college student.
“That’s the No. 1 selling point,” said Teneia Sanders Eichelberger, who plays in a husband-wife band here and supported Mr. Sanders in 2016. “For me and for my grandmother, who’s 82, she loved that about him.”
But unless they already knew about Mr. Sanders’s connection to the movement, hundreds of would-be Democratic primary voters left the gathering none the wiser. (Mrs. Clinton won the 2016 Mississippi Democratic primary with nearly 83 percent of the vote; Mr. Sanders took 16.5 percent.)
Part of Mr. Sanders’s appeal is that he is not a typical, lip-biting politician, ever on the lookout to find a personal connection with any audience. But his relentless focus on the policy dimensions of social justice, which has been the animating cause of his life, can also deprive him of creating bonds that can be essential, especially in building a multiracial coalition.
“Yes, I’m a fairly private person and I don’t like to talk about every aspect of my life,” Mr. Sanders acknowledged in a dressing-room interview after the forum. “I think a lot of politicians do that in a way that is not appropriate.”
Upon hearing the suggestion that recounting his own youthful activism would be compelling to an audience full of younger voters becoming activists in their own right, he all but rolled his eyes.
“Somebody might be interested in what I did 50 years ago, that’s fine,” Mr. Sanders said with an evident lack of enthusiasm. “Or what I did yesterday. But what people have got to start focusing on is not me. It’s how we transform America.”
Mr. Sanders’s reticence can frustrate even his closest supporters.
“If you’re talking to a black audience, you’ve got to say, ‘I was fighting for fair housing in the ’60s,’” said Representative Keith Ellison of Minnesota, a top Sanders surrogate in 2016, noting that he has “an interesting story to tell.”
Several of those at the forum Wednesday night said they liked what they heard. And as is typical for Mr. Sanders, who in 2016 did best among millennials, the younger black attendees were the most enthusiastic.
“To hear his voice and see what he stands for, it’s powerful,” said Cassandra Hogue, 26, who backed Mrs. Clinton two years ago as part of what she called “a legacy thing” for the Clintons but said she would be open to supporting Mr. Sanders in 2020.
Deterrian Jones, 19, made the two-hour-plus drive from the University of Mississippi to Jackson and clutched a handful of buttons he bought from vendors outside, one of which featured Mr. Sanders’s unmistakable visage and logo but with a new slogan: “Hindsight 2020.”
“He talks to millennials, unlike other politicians,” Mr. Jones said.
Yet Mr. Sanders could encounter trouble among black voters if he faces a black Democrat in the primary. “Black voters take special pride in being able to vote for viable African-American candidates,” said former congressman Mike Espy, who plans to run for the Senate.
And while few in attendance at the forum said it so directly, many alluded to Mr. Sanders’s age — he is 76 — and voiced a desire for new blood.
“It’s really time for change,” said Rachael Ighoavodha, 24, a recent Jackson State University graduate sporting a “Black Girl Magic” pin. “It’s time for something new.”
In the interview, Mr. Sanders repeatedly assailed what he called the media’s excessive focus on personality over substance. But when confronted with questions about his age, he replied with good-natured humor on a process question.
”What did you say?” he said, feigning hearing loss and gripping a reporter by the shoulder. “Get me my cane.” Yes, age is a fair question, he said. “But health is a factor,” Mr. Sanders quickly added, before turning to an aide and asking how many times the senator had missed work because he was ill.
The staffer could not recall a single instance.