It was all too much for Majority Leader Mitch McConnell (R-Ky.), 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, as The Washington Post’s Paul Kane and Ed O’Keefe reported.

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The mechanism used to silence Warren is known as Rule 19, an arcane and seldom invoked provision in the rules of the Senate. The rule states that senators may not “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.”

What, exactly, 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.

One thing’s for sure, however: The circumstances surrounding Rule 19’s creation were quite different than Tuesday’s exchange on the Senate floor.

Since its founding, the Senate has maintained an evolving list of rules governing civility and decorum in the chamber. As vice president, Thomas Jefferson included 10 rules in his Manual of Parliamentary Practice that dictated how senators were to behave.

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“No one is to disturb another in his speech by hissing, coughing, spitting, speaking or whispering to another,” reads one passage in the manual, “nor to stand up or interrupt him; nor to pass between the Speaker and the speaking member; nor to go across the chamber, or to walk up and down it, or to take books or papers from the [clerk’s] table, or write there.”

Those rules were published in 1801. The incident that paved the way for Rule 19 came more than a century later.

It was February 1902 and a feud was escalating between the two Democratic senators from South Carolina. Benjamin Tillman, the senior senator and something of a political boss in the state, had grown angry that John McLaurin, his protege, was allowing Senate Republicans to court him on some issues, including the annexation of the Philippines.

Furious that McLaurin was colluding with the other side of the aisle, Tillman used a Feb. 22, 1902, speech on the Senate floor to harangue the younger senator. Gesturing toward McLaurin’s empty chair, Tillman accused his counterpart of treachery and corruption, saying he had succumbed to “improper influences,” according to a Senate history of the dispute.

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When McLaurin caught wind of Tillman’s remarks, he rushed into the chamber and shouted that Tillman was telling a “willful, malicious and deliberate lie.”

A fistfight erupted. As Senate historians recounted, “The 54-year-old Tillman jumped from his place and physically attacked McLaurin, who was 41, with a series of stinging blows. Efforts to separate the two combatants resulted in misdirected punches landing on other members.”

When the fight ended, the Senate voted to censure the two men. A panel found that their behavior was “an infringement of the privileges of the Senate, a violation of its rules and derogatory to its high character, tending to bring the body itself into public contempt.”

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The episode prompted the senate to tighten its rules governing decorum in floor debate. Rule 19 (sections 2 and 3, to be precise) was adopted later that year.

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In the time since, the rule has rarely come up. One instance flagged by Bloomberg’s Greg Giroux occurred in 1979, when Sen. Lowell Weicker (R-Conn.) called Sen. John Heinz (R-Pa.) “an idiot” and “devious” in a debate on the Senate floor. Heinz reportedly stormed to the front of the room with a rule book and showed him Rule 19. Majority Leader Robert Byrd (D-W.Va.) defused the situation and asked them to shake hands. Other examples are hard to come by.

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

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Warren seemed taken aback.

“I’m simply reading what she wrote about what the nomination of Sessions to be a federal court judge meant and what it would mean in history for her,” Warren said. The letter said Sessions “lacks the temperament, fairness and judgment to be a federal judge,” and accused him of pursuing a “shabby” voter fraud case against African American activists when he was a prosecutor.

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“You stated that a sitting senator is a disgrace to the Department of Justice,” responded Sen. Steve Daines (R-Mont.), who was presiding during the speech.

About 25 minutes later, McConnell came in and said her quotes from King crossed the line. She was ordered to sit down.

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“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.”

As the exchange spread on social media, some were quick to point out that McConnell was recently the target of a personal attack on the Senate floor. In 2015, Sen. Ted Cruz (R-Tex.) accused him of lying to his colleagues and the press, saying “he is willing to say things that he knows are false.” There was no Rule 19 invocation then.

The vote against Warren means she’ll be barred from speaking further in the floor debate over Sessions’s nomination. But that didn’t stop her from reading the text of King’s letter and streaming it live Tuesday night.

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