Over the weekend, Mr. Glover toured rural communities in South Carolina on behalf of Mr. Sanders, pitching black voters in Hartsville, Dillon and Mullins on his vision of a political revolution.

At times, he related to the audience, often made up of older black voters, in ways Mr. Sanders would find hard to replicate, referencing his upbringing in the A.M.E. church or his family’s roots picking cotton in the South. With Mr. Glover speaking, Mr. Sanders’s slogan of “not me, us” meant something different. It was targeted directly at black people.

At the stop in Sumter, Mr. Glover told a crowd that Mr. Sanders was the 2020 embodiment of Dr. Martin Luther King Jr.’s political ideology.

“If Dr. King was with us today, at 90 years old last year, he would be voting for Bernie Sanders,” Mr. Glover said.

In a later interview, Mr. Glover questioned whether celebrities have extraordinary persuasive power.

“I honestly don’t know, but I don’t abdicate my responsibility as a citizen because I make movies and people like to go to my movies,” he said. “And there are other people supporting him I know — Mark Ruffalo, Susan Sarandon — we can’t wait to get together and campaign for Bernie.”

The 2020 candidates are all seeking political and celebrity endorsements, and proudly announcing them when they land, but the value of big-name fans has not yet seemed to translate into broader support.