“It should be a broader conversation,” said Rashad Robinson, president of Color of Change, a progressive civil rights advocacy group. “It should be a conversation about who gets a say in this country and what we have done to prevent people from having a say in all sorts of ways.”

Felony disenfranchisement laws have a disproportionate impact on black Americans, who are at least four times more likely to lose their voting rights because of them than the rest of the adult population, according to the Sentencing Project. Mr. Sanders struggled to win over black voters in 2016, and his campaign is working to do better this time around.

Representative Alexandria Ocasio-Cortez, a progressive Democrat from New York, sought to reframe the debate over the issue.

“Instead of asking, ‘Should the Boston Bomber have the right to vote?,’” she wrote on Twitter, “Try, ‘Should a nonviolent person stopped w/ a dime bag LOSE the right to vote?’”

That question, she wrote, applies to “WAY more people.”

Critics, meanwhile, said Mr. Sanders may have crossed a line.

“There’s a difference between felons who are in prison for nonviolent offenses and those who are in for capital offenses,” said Jim Kessler, executive vice president for policy at Third Way, a center-left think tank. Mr. Sanders’s remarks, he added, “seemed like a bit of a stumble.”

While the exact number of convicted felons in the country is hard to pin down, a 2016 report estimated that 6.1 million Americans had been barred from voting because of felony disenfranchisement laws. Christopher Uggen, a professor of sociology and law at the University of Minnesota who was one of the authors of the report, estimated that about a quarter of those people were incarcerated.