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Warren, D-Massachusetts, was quoting the late Sen. Ted Kennedy's remarks from Sessions' 1986 hearing for a federal judgeship -- part of a series of round-the-clock speeches during the confirmation process of Trump's nominees.
"He is, I believe, a disgrace to the justice department and he should withdraw his nomination and withdraw from the Justice Department… Like he did, I will cast my nomination against the vote of Sen. Sessions,” Warren said, quoting Kennedy.
During the 1986 hearings, Sessions faced scrutiny over allegations that he made racist remarks as a U.S. attorney in Alabama. But Sessions has repeatedly said the allegations are false. "I hope my tenure in this body shows you that the caricature of me wasn't accurate," Sessions told the Senate last month. "It wasn't accurate then, and it's not accurate now."
Sen. Steve Daines, R-Montana, the Senate's presiding officer, warned Warren that senators cannot “directly or indirectly, by any form of words impute to another Senator or to other Senators any conduct or motive unworthy or unbecoming a Senator," he said.
"The senator has impugned the motives and conduct of the senator from Alabama,” McConnell said. After that, Daines told Warren to "take her seat," he said.
"She was warned," McConnell said referring to Warren. "She was given an explanation. Nevertheless, she persisted."
A majority of senators voted to deny Warren's appeal, effectively silencing her for the rest of the session under the Senate's arcane rule XIX.
Democratic National Committee Chair Donna Brazile blasted the move: "It’s a sad day in America when the words of Martin Luther King Jr.’s widow are not allowed on the floor of the United States Senate. Let Elizabeth Warren speak. The American people deserve to hear how Jeff Sessions is an extremist who will be a rubber stamp for this out-of-control Trump presidency.”
Republicans maintain that Warren was rebuked for quoting Kennedy, not King.
"I just want the record to be abundantly clear. The language that resulted in the vote that we had invoking rule XIX was related to a quotation from Sen. Ted Kennedy that called the nominee a disgrace to the Justice Department and he should withdraw his nomination and resign his position," Cornyn said. "That was the quote. Our colleagues want to try to make this all about Coretta Scott King and it is not. I think the complete context should be part of the record."
But the actual censure of Warren occurred during her reading of King's letter, and she maintains that was the proximate cause of her being silenced.
"Senator Mitch McConnell and the Republicans came to the floor to shut me down for reading that letter," Warren said in a subsequent video from outside the Senate chamber that she posted on her Facebook page. "So what I would like to do, outside of the Senate, I just want to read the letter," she said, before preceding to read the letter in its entirety.
The hashtag "#LetLizSpeak" was trending on Twitter after social media users rallied to her defense, while Warren’s Facebook video racked up nearly 3 million views.
Senator Cory Booker, D-New Jersey, defended Warren in a speech from the Senate floor on Tuesday. He said he believes that the rule that silenced her is "selectively enforced."
"You see Senator Warren stood up and was speaking out with passion about this nomination and in the mist of her speaking her truth in the mist of her speaking her heart, she was stopped as she read something into the record that has been there for decades," Booker said.
Warren "was stopped because of a rule being enforced that in my opinion as well as Leader Schumer is selectively enforced," he continued.
ABC News' Karma Allen contributed to this report.