“The senator has impugned the motives and conduct of our colleague from Alabama,” McConnell said from the Senate floor. “I call the senator to order under the provisions of Rule 19.”
Here’s #2 of Rule 19 (XIX) of the Rules of the Senate:
No Senator in debate shall, 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.
And, how was bringing up and reading from the letter of Coretta Scott King considered “impugning a fellow senator”? The letter itself is in the words of Coretta Scott King and not Senator Warren.
“The senator is reminded that it is a violation of Rule 19 of the standing rules of the Senate to impugn another senator or senators any conduct or motive unworthy or unbecoming a senator,” said Sen. Steve Daines, R-Montana, who was presiding over the Senate at the time.
Here is the video and transcription from C-Span: https://www.c-span.org/video/?c4655562/senator-elizabeth-warren-blocked-speaking-senate-floor-sessions-remarks
Letter of Coretta Scott King in 1986 against Jeff Sessions’s federal nomination (Source):