After the vote to bar Ms. Warren from speaking further about Mr. Sessions, other senators, including Bernie Sanders of Vermont and Tom Udall of New Mexico, read Mrs. King’s letter without facing any objection, prompting some activists to raise charges of sexism.

Ms. Warren has long displayed an instinct for capitalizing on highly visible fights. After she was barred from speaking on the Senate floor, she began reading the 1986 letter from Mrs. King on Facebook. By Wednesday evening, the video had attracted more than nine million views.

In the letter, Mrs. King, the widow of the Rev. Martin Luther King Jr., took aim at Mr. Sessions’s record on civil rights as a United States attorney in Alabama, saying he had used “the awesome power of his office to chill the free exercise of the vote by black citizens.” She called on the Senate not to confirm Mr. Sessions to a federal judgeship, and his nomination to that post was ultimately rejected.

On Wednesday morning, in a conference room in the Capitol — the vote prohibited Ms. Warren from speaking about the nomination only from the Senate floor — Ms. Warren addressed civil rights leaders, recounting her long night.

“What hit me the hardest was, it is about silence,” she said. “It’s about trying to shut people up. It’s about saying, ‘No, no, no, just go ahead and vote.’”

She went on.

“This is going to be hard,” she said. “We don’t have the tools. There’s going to be a lot that we will lose. But I guarantee, the one thing we will not lose, we will not lose our voices.”