WASHINGTON — America got a civics lesson Tuesday night when Senate Republicans used an obscure rule to shut down a speech by Sen. Elizabeth Warren, D-Mass., that criticized Sen. Jeff Sessions, R-Ala., the nominee for attorney general.Republicans took issue when Warren quoted from a pair of letters written by the late Coretta Scott King and the late Sen. Edward Kennedy, D-Mass., opposing Sessions's 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 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.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 does it mean for one senator to "impugn" or "impute" another? That's a matter of perspective, as congressionalDemocrats 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."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."The incident that paved the way for Rule 19 came more than a century after the first rules were published.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 issues relating to trade and 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. He accused McLaurin of treachery and said he had succumbed to "improper influences," according to a Senate history of the dispute.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-four-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, with a panel finding 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."The episode prompted the senate to tighten its rules governing decorum in floor debate. Rule 19 (sections two and three, to be precise) were adopted later that year.In the time since, the rule has seldom come up. One instance flagged by Bloomberg's Greg Giroux came in 1979, when Sen. Lowell Weicker, R-Conn., called Sen. John Heinz, R-Pa., "an idiot" and "devious," causing then-majority leader Sen. Robert Byrd, D-W.Va., to defuse the situation. Other examples are hard to come by.In Warren's case, 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.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."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."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 once the target of a personal attack on the Senate floor. In 2015, Sen. Ted Cruz , R-Texas, accused him of lying to 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 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."I am surprised," Warren said, "that the words of Coretta Scott King are not suitable for debate in the United States Senate."