"Last night we all witnessed a rather extraordinary event," said Sen. John Cornyn, R-Texas, said after a heated debate and roll call vote that determined Warren had violated Senate Rule 19, that prohibits a senator from impugning another while on the floor. "I certainly agree with the ruling of the chair -- and the decision of the Senate as a body -- that line was crossed last night."
The rule is so rarely used the Senate Historian's Office scrambled to determine when it was last enforced. Sen Orrin Hatch, the longest serving Republican, said it had never happened in his forty years on the job defended it.
"We tolerated the speech, which was a harangue against Jeff Sessions," he said about Warren's speech. "Usually we as fellow senators don't treat fellow senators that way. That bothered me a lot."
Despite being obscure, Republicans had discussed privately the Rule 19 issue before the floor debate on Sessions started and actively monitored the debate for infractions once it began, Senate GOP leadership aides told CNN. They knew tensions in the chamber were high already because of long drawn out fights over President Donald Trump's Cabinet members and the Democrats were swinging for Sessions, who has been accused of racism decades ago when he was a prosecutor in Alabama.
Warren was raising those issues in her speech, quoting strong criticism raised against Sessions' 1986 nomination to be a federal judge by former Sen. Ted Kennedy, D-Massachusetts, and civil rights leader Coretta Scott King.
Republicans knew Sessions had been pummeled during his confirmation hearings by Democrats, who also routinely blasted him at news conferences and at other events. Republicans were worried Sessions could be damaged if the hot rhetoric continued on the floor. When Republican leaders stepped in to cut Warren off, they did it part because they feared a long list of Democrats were about to take the floor and blast Session, the aides said.
Republicans also chaffed at the idea that the Democrats would fire up their base and raise money for their campaigns at the expense of Sessions, who Republican senators universally admire as a principled senator and genuinely polite man. Republicans believe that until this point, they have held back from engaging heavily with Democrats on their attacks but couldn't standby quietly anymore.
Aides also point out that Senate Majority Leader Mitch McConnell and other Republicans would have been battered by outside voices on the right had they not come to Sessions' defense.
"There are rules about what we can say about our personal opinion about other members of the Senate," said Republican Sen. Roger Wicker of Mississippi. "She broke those rules, she was advised in a very correct manner by the chair and she persisted and we really had no choice but to enforce those."
'Unbecoming a Senator': What the rule says
Rule 19 of the Senate rules states that "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 same rule prohibits a senator from referring "offensively to any State of the Union." And a senator that violates one of these provisions "shall take his seat," just as Warren late Tuesday was ordered by the presiding officer to sit down.
Democrats rushed to Warren's defense -- on the Senate floor and on social media -- and accused Republicans of unfair "selective enforcement" of the rule.
"The Rule is only intended to keep Senators on the facts; to keep them from making baseless accusations about another's character," said Senate Minority Leader Chuck Schumer, D-New York. "My friend from Massachusetts was following the letter and spirit of the rule last night. She was engaging in that tradition of forceful but respectful debate when she was cut off."
McConnell knows the risks of enforcing Rule 19. When Sen. Ted Cruz, R-Texas, called him a liar on the floor in 2015, McConnell urged his fellow Republicans not to use the tool against the junior senator fearing Cruz could appear to be a martyr -- a fate that now seems to benefiting Warren, a potential presidential candidate in 2020.