Elizabeth Warren
Sen. Elizabeth Warren (D-Mass.), a member of the Senate Banking and Urban Affairs Committee, listens to the testimony by Federal Reserve Board Chair Janet Yellen as she delivers her semi-annual report on monetary policy, on Capitol Hill in Washington, D.C., July 16, 2015. REUTERS/Mike Theiler

Sen. Elizabeth Warren (D-Mass.) inspired a hashtag to go viral Tuesday night after she was silenced by The Senate. "Nevertheless, she persisted," and #shepersisted swirled on social media, with photos of women who have refused to give up.

The viral trend was a response to Senate Majority Leader Mitch McConnell invoking an archaic rule, Rule 19, to stop Warren from reading a 1986 letter from the late Coretta Scott King opposing Jeff Sessions. Sessions, now a Republican senator from Alabama, has been nominated to become attorney general, and Warren is one of the Democrats who have sharply criticized the pick.

McConnell's action sparked a debate: Should he have been able to silence Warren? That depends on your interpretation of Rule 19, which 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 of a Senator.”

What exactly is “unworthy” or “unbecoming” of a senator? For critics, that’s the trouble with this rule — senators interpret those guidelines differently. When Warren appealed the rule with a roll call vote, Senators were split on a party line vote. Republicans sided with McConnell. Democrats sided with Warren.

The rule is over a century old, which means it comes from a time — and culture — very different from today. In fact, the original 10 Senate rules were established at an even earlier time: in 1801, when Thomas Jefferson published guidelines for decorum on the Senate floor called A Manual of Parliamentary Practice.

“No one is to disturb another in his speech by hissing, coughing, spitting, speaking or whispering to another,” one passage states.

Rule 19 wasn’t added until 1902, when a fistfight broke out between two senators. More senators tried to stop the fight, but they ended up falling on one another and getting punched as well, Senate historians said. It had turned into a full-on, albeit unintentional, brawl on the Senate floor.

Rule 19 has only been used a handful of times since then.

Warren seemed startled that her letter reading violated Senate decorum.

“I am surprised that the words of Coretta Scott King are not suitable for debate in the United States Senate," she said.