Sen. Elizabeth Warren (D-Mass.) was stopped from speaking on the Senate floor about Attorney General nominee Jeff Sessions on Feb. 7. "I am surprised that the words of Coretta Scott King are not suitable for debate in the United States Senate," Warren said. (Reuters)

On Tuesday night, Republicans handed Sen. Elizabeth Warren (D-Mass.) the equivalent of a red card when they voted along party lines that she violated Rule 19, a century-old rule prohibiting senators from insulting each other on the hallowed Senate floor. The offending moment came when Warren read a letter from Martin Luther King Jr.'s widow declaring Sen. Jeff Sessions (R-Ala.), President Trump's pick for attorney general, "used the awesome power of his office to chill the free exercise of the vote by black citizens."

Warren is the rare senator to be caught up in this rule. Since its inception  after an actual fist fight on the Senate floor a century ago, Rule 19 has been used more as a threat to keep senators in line than a practice. Historians and Senate aides have an easier time coming up with moments when it perhaps should have been invoked but wasn't, than when it actually was invoked.

There are a few reasons for that. Rule 19 is actually sort-of tricky. Like a referee watching for fouls, calling someone out on Rule 19 requires a senator to declare a violation in real time -- and then for a majority of senators to vote in agreement that said senator violated it. In many recent instances where Rule 19 could have been invoked, Senate leaders have decided going through all that trouble just wasn't worth it.

The fact this rule is used to rarely opens up Republicans who used it to some predictable criticism from Democrats. Senate Minority Leader Chuck Schumer (D-N.Y.) said on the Senate floor on Tuesday he didn't think Warren deserved to be rebuked.

Republicans say this time and this moment was worth the trouble. Senate Democrats had spent the past month attacking Sessions as, among other things, hostile to civil rights. Sen. Cory Booker (D-N.J.) even broke with precedent and testified against Sessions at his high-profile confirmation hearing. In many ways, Sessions's contentious nomination epitomizes the breakdown of respect for different views on both sides of the aisle.

As the full Senate launched into several days of debate on Sessions's, Republicans were on high alert for more insults lobbed at Sessions, Rule 19 at the ready. They got their moment with Warren. (It doesn't hurt that Warren is both high profile and reviled by the Republican base.)

But here are four other moments where Rule 19 arguably could have been used, but, for various reasons, wasn't.

1. Sen. Tom Cotton (R-Ark.) decries Harry Reid's "cancerous leadership"

Sen. Tom Cotton (R-Ark.) tore into Senate Minority Leader Harry Reid (D-Nev.) on the floor of the Senate on May 25, calling his leadership "cancerous." (C-SPAN)

It's May 2016. Republicans are in charge of the Senate, and Democrats have stalled a defense bill over concerns it was written too quickly and without their input. Cotton, a freshman senator with a little more than a year of experience under his belt, smashed through decorum to blast then-Senate Minority Leader Harry Reid (D-Nev.) and his "cancerous leadership."

"I'm forced to listen to the bitter, vulgar, incoherent ramblings of the minority leader," "Normally, like every other American, I ignore them. I can't ignore them today. . . . When was the last time the minority leader read a bill? It was probably an electricity bill. ... This institution will be cursed less with his cancerous leadership."

Why he wasn't called out for violating Rule 19: It's unclear. It's possible Democrats either weren't on the floor at that time, or they just didn't think it was necessary or worth the trouble.

We do know Reid took matters into his own hands when he came back on the Senate floor and responded to Cotton himself: "I think it would distract from what we're doing here today to go into the statements by the very junior senator from Arkansas."

Republicans counter that what Cotton did -- a one-off speech to blow off some steam -- was different than what Warren did, where she continued to criticize Sessions after being warned by Republicans that she was in danger of violating the rule.

2. Sen. Ted Cruz (R-Texas) called McConnell a liar

On the Senate floor July 24, Sen. Ted Cruz (R-Tex.) accused "career politicians" in Congress of "looting the tax payer to benefit wealthy, powerful corporations." (AP)

It's summer 2015. The Republican Party is in the midst of a showdown with itself over whether to fund an obscure government agency that helps fund risky U.S. investment abroad, the Export-Import Bank. As I reported at the time:

Cruz got wind that Senate Republicans were going to try to start up the bank as part of a three-month transportation funding bill. He marched onto the Senate floor and accused Senate Majority Leader Mitch McConnell (R-Ky.) of lying to him -- a pretty incredible breach of Senate decorum.

"What we just saw today was an absolute demonstration that not only what he told every Republican senator, but what he told the press over and over and over again was a simple lie," Cruz said."We now know that when the majority leader looks us in the eyes and makes an explicit commitment, that he is willing to say things that he knows are false."

This portion has been corrected to clarify that Cruz never actually directly called McConnell a "liar" on the Senate floor.

Why he wasn't called out for violating Rule 19: This appeared to be a battle that Republicans didn't want to play out publicly on the Senate floor. Well-sourced Politico reporter Burgess Everett says McConnell convinced other senators to stand down.

Instead the matter was dealt with behind closed doors, where Republican senators strongly rebuked their colleague.

3. Reid calls GOP senators "puppets"

It's 2007. Democrats are in control of the Senate, and newish Senate majority leader Reid proves his new title won't stop him from saying controversial things that make other senators blanch.

President George W. Bush, Reid argued, "is the man who is pulling the strings on the 49 puppets he has here in the Senate. That is too bad for the country."

Why he wasn't called out for violating Rule 19: Actually, former Sen. Arlen Specter (R-Pa.) tried. He was on the Senate floor at the time and launched his own speech.

"I wonder if he is up to the job when he resorts to that kind of a statement, which only furthers the level of rancor and insults and animosity with that kind of an insulting comment," Specter said of Reid, according to the Las Vegas Sun. Specter also suggested Reid had violated Rule 19 of the Senate. But he didn't push the matter further.

4. 1979: A senator calls a colleague "an idiot" and "devious"

Bloomberg's Greg Giroux dug up this moment decades ago, when Sen. Lowell Weicker (R-Conn.) called Sen. John Heinz (R-Pa.) “an idiot” and “devious." The Post's Derek Hawkins reports Heinz reportedly stormed to the front of the room with a rule book and showed him Rule 19.

Why he wasn't called out for violating Rule 19: Once again, Senate leaders didn't want to go through the trouble. Hawkins reports Majority Leader Robert Byrd (D-W.Va.) asked them to shake hands.