State voters jettisoned a 120-year-old mainstay of Louisiana justice Tuesday, overwhelmingly passing a constitutional amendment to require unanimous juries in all felony trials and ending an aberrant practice rooted in the Jim Crow era.

Passage of Amendment 2 to the state constitution means future juries must come to unanimous verdicts in serious felony cases — from second-degree murder to marijuana distribution — or else declare themselves deadlocked. Unanimity already was required in capital trials and those for lesser felonies decided by six-member juries. 

Since 1974, a verdict — for conviction or acquittal — has been considered valid as long as 10 of 12 jurors agreed. In the law’s original 1898 conception, just nine votes were required. Before 1898, Louisiana required complete consensus.

Louisiana’s rejection of split verdicts, the product of a remarkable bipartisan coalition, leaves Oregon as the only state not to require unanimity in felony cases.

The change to the law won’t take effect immediately. It will apply only to trials involving crimes committed on or after Jan. 1, 2019. Given that felony cases typically take months and sometimes years to try, Louisiana could still be churning out split verdicts for a couple more years, even while the new rules come into play.

The road to Tuesday’s vote was a remarkable one, requiring that a resolution to put the measure on the ballot pass both chambers of the state Legislature by a two-thirds margin.

The bill’s primary sponsor, state Sen. JP Morrell, D-New Orleans, didn’t give the initiative much of a shot when he introduced it in March. Not since the ratification of the current constitution in 1974 had there been a serious push to change the law.

Changing it had long been advocated by liberal groups such as the American Civil Liberties Union, mainly because the 10-of-12 system generally makes it easier for prosecutors to win convictions.

But Morrell was joined early on by some conservatives, including Ed Tarpley, the former district attorney for rural Grant Parish, who invoked the framers in their framing of the matter. Tarpley and others noted that unanimity had been required in jury verdicts for hundreds of years in Anglo-American jurisprudence and argued that a high bar for conviction was needed to check prosecutors’ formidable power to deprive citizens of liberty.

As debate on the issue began, The Advocate published the results of a yearlong investigation that found that 40 percent of convictions in Louisiana jury trials came with one or two dissenters. According to the newspaper’s data, black defendants — who make up nearly two-thirds of inmates in the state — were 30 percent more likely than white ones to be convicted by split jury verdicts.

Moreover, the newspaper found that black people were significantly underrepresented on juries, and black jurors were 2½ times more likely than white ones to dissent with a conviction.

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The powerful Louisiana District Attorneys Association, representing the state’s 42 district attorneys, initially took a strong stance against the proposal. But then the association — after internal discussions — decided to remain neutral, removing what could have been a strong countervailing force.

And suddenly the measure began to gain momentum. After narrowly passing the Senate, it easily cleared a House committee and then passed the full House by a much wider margin than it needed. 

The key was a bipartisan coalition that developed during the session. The liberals who backed the bill initially were joined by Republicans, including state Rep. Sherman Mack, R-Albany, who carried the bill in the House, and conservative groups like the Louisiana Family Forum and Americans for Prosperity.

The coalition resembled the one that had formed a year earlier to pass the Justice Reinvestment Act, a reform package intended to reduce Louisiana’s world-leading incarceration rate and ease ex-convicts’ transition back into civilian life.

Eventually, a handful of district attorneys — including those from Louisiana's two biggest parishes — came out in support of unanimous verdicts.

“A hundred years of nonunanimous juries has not been a beneficial thing for Louisiana. It’s time for that change to happen,” said Morgan Clevenger, who cast a vote about 7 p.m. at St. Peter’s Presbyterian Church on North Dorgenois Street in the 6th Ward.

Clevenger said she was impressed with the effort to educate voters on the amendment, something she said she’d like to see replicated on other, often confusing ballot measures. She also found cause for hope in the bipartisan consensus around the ballot measure.

“When people are at such extremes — when people can come together around something like this, that is quite a good thing,” she said. “It is a big deal to be one of two states to have this 100-year-old law that doesn’t seem fair.”

Ben Cohen, an attorney with the Promise of Justice Initiative who has filed numerous legal challenges to the state's split verdict law at the U.S. Supreme Court, harkened to the work of New Orleans civil rights attorney Louis Martinet, whose pleas for the U.S. attorney general to investigate “Jim Crow juries” in the late 1800s went unfulfilled.

“This last year has been us trying to make good on our failed promises,” Cohen said. “It’s been a recognition from white and black, prosecutor and defense lawyer, Republican and Democrat, Catholic and Jew, that our rights and liberties matter (and) hold us together as a civilized society.”

An early analysis of voting patterns from Tuesday’s election suggests that liberals overwhelmingly supported Amendment 2 while conservatives offered less-monolithic but still robust support in a state where Donald Trump took 58 percent of the vote in 2016. 

There was a rough correlation between the 2016 presidential contest and the vote on Amendment 2, with parishes won by Hillary Clinton tending to support the measure by wider margins. For instance, in Orleans Parish, where Clinton won 81 percent of the vote in 2016, 85 percent of voters supported Amendment 2 on Tuesday. And in tiny Lasalle Parish, where Clinton got just 9 percent of the vote, 40 percent of voters endorsed Amendment 2 — the worst result the measure saw in any parish.

The amendment also appeared to pass more narrowly in the rural northern parishes of the state than in the south. But in the end, 61 of 64 parishes voted to put an end to the controversial vestige of another time.

Follow John Simerman on Twitter, @johnsimerman.