Rule 19 forbids lawmakers in the Senate from "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 rule was created in 1902 after a fist fight actually broke out in the chamber between two South Carolina senators.
"On February 22, 1902, John McLaurin, South Carolina's junior senator, raced into the Senate Chamber and pronounced that state's senior senator, Ben Tillman, guilty of "a willful, malicious, and deliberate lie." Standing nearby, Tillman spun around and punched McLaurin squarely in the jaw. The chamber exploded in pandemonium as members struggled to separate both members of the South Carolina delegation. In a long moment, it was over, but not without stinging bruises both to bystanders and to the Senate's sense of decorum."- Senate's official website.
The fight spurred the creation of Rule 19, but it has seldom been used since and that isn't because lawmakers have acted with such decorum for more than 100 years. It wasn't long ago that junior Sen. Ted Cruz (R-TX) went to the floor and called his leader, McConnell, a liar. Rule 19 wasn't used then. Nor was it ever invoked over a period of testy floor speeches between former Senate Minority Leader Harry Reid (D-NV) and McConnell.
The use of the rule Tuesday was jarring for congressional experts like Norm Ornstein.
"It is rarely applied," Ornstein said. "The whole thing struck me as peculiar."
Ornstein argued that what was strange to him was that Warren wasn't talking about Sessions in his official capacity as a senator, she was discussing him in his capacity as a cabinet nominee. Warren also was reading from a letter, not speaking out against him with her own words.
"It was an odd thing, and I don't know whether it was an attempt to get at Warren because he does not like Warren or to show who is boss or out of frustration because these confirmations have been going on so long," Ornstein said.
The last use of Rule 19 appeared to be in 1979, according to a Bloomberg reporter who'd pulled the congressional record on the incident.
In that incident, the reporter recounted that "John Heinz (R-PA) invoked Rule XIX after Lowell Weicker (R-CT) called him 'an idiot' and 'devious.'" There was never a vote on the rule, however, because Sen. Robert Byrd "worked out a truce."
1979: John Heinz (R-PA) invoked Rule XIX after Lowell Weicker (R-CT) called him “an idiot” and “devious.” Robert C. Byrd worked out a truce pic.twitter.com/G3UEOcDPjF
— Greg Giroux (@greggiroux) February 8, 2017