In the letter, the wife of the late Rev. Martin Luther King Jr. had opposed Sessions' failed bid for a federal judgeship. As a U.S. attorney, King wrote, Sessions had "used the awesome powers of his office in a shabby attempt to intimidate and frighten black voters."
Within hours of the Republican majority's silencing of Warren for violating a Senate rule against impugning the motives of a fellow senator, Warren supporters had made a meme of McConnell's justification for shutting her up: "She was warned; she was given an explanation. Nevertheless she persisted."
The rule states: "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."
The original intent of the rule, if you will, was to preserve comity and focus the attention senators on substance rather than ad hominem arguments. But Warren was commenting on Sessions not as a colleague but as the nominee to a position in the executive branch; his character (as perceived by Mrs. King) was central to her argument.
McConnell should have let this slide.