Sen. Elizabeth Warren stepped onto the Senate floor last night with a simple plan: Read a letter by the late civil rights icon Coretta Scott King criticizing attorney general nominee Jeff Sessions. Given Sessions' incendiary record on racial justice, King's opinion was quite relevant to the debate on whether he should be America's top law enforcement official.

But the GOP majority didn't want to hear King's letter. So they surprisingly invoked a 115-year-old rule never before used in the Senate to shut Warren down. And Miami's proudest son, Marco Rubio, joined his GOP colleagues in voting to silence Warren.

Rubio framed his vote as a mark for civility in politics. "This body cannot function if people are offending one another, and that's why those rules are in place," Rubio said in a floor speech. "This is not a partisan issue. It really is not. If one of my colleagues on this side of the aisle had done that, I'd like to think I'd be one of the people objecting."

What could King have possibly written that would leave Republicans so shaken that they'd dig into their obscure rulebook to muzzle a senator on the floor?

In short: a lot. In ten pages, King eviscerates Sessions' record as a federal prosecutor in Alabama, where he plotted numerous "voting fraud" cases that netted zero convictions but sent the message that poor black residents weren't welcome at the ballot box.

King's letter, which dates to 1986, when Sessions was up for a federal judgeship, says Sessions on the bench would “irreparably damage the work of [her] husband” Martin Luther King Jr. because he would "be given a life tenure for doing with a federal prosecution what the local sheriffs accomplished twenty years ago with clubs and cattle prods.”

Her testimony was powerful enough 31 years ago to sink Sessions' chances at that judgeship. Now the GOP wants to prevent King's letter from entering the debate over his qualifications to be AG.

When Warren began to read the letter last night, Sen. Steve Daines, a Republican from Montana, interrupted her and told her she was violating Rule 19. That's a truly obscure bit of Senate legalese that forbids senators from impugning one another; it was passed in 1902 after a fistfight broke out on the Senate floor but had never really been used before.

Warren appeared stunned and appealed that ruling, and Senate Majority Leader Mitch McConnell called a vote. Senators split along party lines and confirmed that Warren wouldn't be allowed to read the letter.

Yes, the letter is a harsh indictment of Sessions. But it's based on his record as a federal prosecutor, and it's written by a giant in the civil rights movement. It's not as if Warren jumped onto the Senate floor and began dropping F-bombs at Sessions for fun.

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

Of course, Rubio's vote doesn't mean you can't read King's letter in full. Warren took to Facebook Live to read it after being silenced on the Senate floor:

The Washington Post has also published the ten-page letter: