The most recent jarring example of the unraveling of our political culture — and consensus about rules of the game — came in the U.S. Senate. What used to be called “the world’s most deliberative body” is cannibalizing itself.

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Last week, Sen. Elizabeth Warren (D., Mass.) “impugned” her colleague, Sen. Jeff Sessions (R., Ala.), in the debate over his nomination to be attorney general of the United States. She read a statement from Sen. Ted Kennedy in 1986, when Mr. Sessions was nominated to be a federal judge, calling him a “disgrace.” Senate Majority Leader Mitch McConnell invoked Rule 19, which forbids such aspersions. The rule is seldom cited. It was not applied when Sen. Ted Cruz called Mr. McConnell a liar, some months ago. The last time it was even considered was more than 25 years ago, when Sen. Lowell Weicker, of Connecticut, called Sen. John Heinz, of Pennsylvania, “an idiot.”

But Mr. McConnell used the rule to gag Ms. Warren. It backfired politically, of course, as it should have.

We don’t silence U.S. senators on the Senate floor in America. At least we never used to.

Mr. Sessions, a 20-year veteran of the Senate, had been called far worse than an idiot for days and weeks prior to the vote on his nomination. Basically he was called a racist by many of his longtime colleagues. Most of them know him well and know very well that he is no such thing. A Southern conservative and too conservative for most of them to see him as a proper AG? Certainly. But a racist? No.

Yet, Ms. Warren, who should not have been silenced on the Senate floor but shamed instead, has been back at it every day since that awful night when the Senate seemed to be coming apart. Yes, she insists, her former colleague is a racist and unfit.

Part of what made the Senate a great institution (second in dignity only to the Supreme Court) and occasionally a truly deliberative body is that it was the most polite such body in the world. The British House of Commons has never been polite. Nor has the U.N. It would have been inconceivable, just a few years ago, for a veteran of the Senate to have roughly half his colleagues vote against him purely on a partisan basis. Presidents nominated senators because “senatorial courtesy” would get them 90 or more votes, not 52. No senator in the days of Byrd, Ervin, Fulbright, or Russell would have ever called another senator a racist, even if they knew him to be one. Many of the great Southern senators were, in fact, racists.

Marco Rubio, of all people, took to the floor of the U.S. Senate last week and defended civility, indeed defended the Senate itself, where he has only served one thoroughly undistinguished term and where he was frequently absent. His speech was plain, but his sincerity made him eloquent.

He said:

“I don’t know of a single nation in the history of the world that has been able to solve its problems when half the people in a country absolutely hate the other half of people in that country.

“... we are reaching a point in this Republic where we’re not going to be able to solve the simplest of issues because everyone is putting themselves in a corner where everyone hates everybody.”

Finally, he said: “I know this: If this body is incapable of having those debates, there will be no place in this country where those debates can occur.”

Well said, senator. Your words are disturbing and honest.

To watch Mr. Rubio’s full speech, go to: http://​bit.ly/​2kBVObv.

First Published February 14, 2017, 5:00 a.m.