WASHINGTON—Senator Elizabeth Warren earned a rare rebuke by the Senate for quoting Coretta Scott King on the Senate floor, after Republicans used an obscure rule to shut down her speech that criticized Senator Jeff Sessions, the nominee for attorney general.

Republicans took issue when Massachusetts Democrat quoted from a pair of letters written by the late King and the late Sen. Edward Kennedy opposing Sessions' ill-fated nomination to a federal judgeship in 1986. King's letter accused Sessions of racial bias; Kennedy's called him a "disgrace to the Justice Department."

It was all too much for Majority Leader Republican Mitch McConnell, who said Warren had "impugned the motives and conduct of our colleague from Alabama." In an extraordinary move, the Senate voted on party lines to shut her down.

King wrote that when acting as a federal prosecutor, Sessions used his power to “chill the free exercise of the vote by black citizens.”

Quoting King technically put Warren in violation of Senate Rule 19, for “impugning the motives” of Sessions, though senators have said far worse. And Warren was reading from a letter that was written 10 years before Sessions was even elected to the Senate.

Still, top Senate Republican Mitch McConnell invoked the rules. After a few parliamentary moves, the GOP-controlled Senate voted to back him up.

Now, Warren is forbidden from speaking again on Sessions’ nomination. A vote on Sessions is expected Wednesday evening.

What does it mean for one senator to "impugn" or "impute" another? That's a matter of perspective, as congressional Democrats and other Warren defenders made clear when they rallied behind her on Twitter, launching the hashtag #LetLizSpeak to the top of the site's trending list.

Senate Republicans balked at Warren's use of the word "disgrace," as quoted from the Kennedy letter, in reference to Sessions. But it was during her reading the letter from King, the widow of Martin Luther King Jr., that Republicans warned her that she was violating Rule 19.

"Sen. Warren was giving a lengthy speech. She had appeared to violate the rule. She was warned. She was given an explanation," McConnell said later. "Nevertheless, she persisted."

Warren argued: “I’m reading a letter from Coretta Scott King to the Judiciary Committee from 1986 that was admitted into the record. I’m simply reading what she wrote about what the nomination of Jeff Sessions to be a federal court judge meant and what it would mean in history for her.”

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Warren was originally warned after reading from a statement by former Sen. Edward Kennedy that labeled Sessions a disgrace.

"I am surprised," Warren said, "that the words of Coretta Scott King are not suitable for debate in the United States Senate."

Democrats pointed out that McConnell didn’t object when Sen. Ted Cruz called him a liar in a 2015 dustup.

The episode was followed by lamentations by Senate veterans, including its most senior Republican, Orrin Hatch of Utah, about how the Senate is too partisan.

— With files from Washington Post