Laurence Tribe, a professor of constitutional law at Harvard University, warned on Wednesday that the U.S. was heading down a slippery slope toward autocracy after Senate Majority Leader Mitch McConnell (R-KY) used Senate rules to silence Sen. Elizabeth Warren (D-MA).

During a Senate debate over Jeff Sessions’ confirmation for attorney general Wednesday night, McConnell invoked Senate Rule 19 to silence Warren while she was reading a letter written by the widow of Martin Luther King, Coretta Scott King, about Sessions 30 years ago. The rule says senators cannot “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.”

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On Wednesday, Tribe suggested that the incident was a troubling sign for democracy.

“When Trump equates our errors with how Putin deals with political opponents, we should reflect on how Sen. Warren’s critique of Jeff Sessions would go down in Putin’s Russia,” Tribe wrote on Twitter. “Then reflect on how McConnell’s outrageous silencing of Warren takes us a dangerous step toward autocracy and a society we won’t recognize unless we stop this scary slide down a very slippery slope.”

He added: “Applying Senate Rule 19 to prevent a Senator from impugning the character of a fellow Senator who is up for confirmation as AG is a perversion of Rule 19, an abdication of the Senate’s Advise & Consent duty, and a threat to DOJ’s role in enforcing the rule of law.”

1/When Trump equates our errors with how Putin deals with political opponents, we should reflect on how Sen. Warren's critique of — Laurence Tribe (@tribelaw) February 8, 2017

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2/ Jeff Sessions would go down in Putin's Russia. Then reflect on how McConnell's outrageous silencing of Warren takes us a dangerous step — Laurence Tribe (@tribelaw) February 8, 2017

3/ toward autocracy and a society we won't recognize unless we stop this scary slide down a very slippery slope — Laurence Tribe (@tribelaw) February 8, 2017

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1/ Applying Senate Rule 19 to prevent a Senator from impugning the character of a fellow Senator who is up for confirmation as AG is — Laurence Tribe (@tribelaw) February 8, 2017

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2/ a perversion of Rule 19, an abdication of the Senate's Advise & Consent duty, and a threat to DOJ's role in enforcing the rule of law — Laurence Tribe (@tribelaw) February 8, 2017