Four states have voted to change their state constitutions to remove a loophole allowing slavery as a form of punishment. 

Voters in Vermont, Tennessee, Oregon, and Alabama voted Tuesday to end constitutional clauses that allow slavery or indentured servitude as a form of punishment.

Ratified in 1865, the 13th Amendment to the U.S. Constitution abolished slavery and involuntary servitude in America, but it provided leeway for the practice as punishment for individuals convicted of a crime. Several state constitutions still have language allowing involuntary servitude for prisoners.

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Advocates for the constitutional changes hope it will help regulate and temper the use of prison labor.

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Tennessee's initiative, which received bipartisan support and is known throughout the state as Amendment 3, will alter the language to read: "That slavery and involuntary servitude are forever prohibited in this State. Nothing in this section shall prohibit an inmate from working when the inmate has been duly convicted of a crime."

On Wednesday, it was announced that Oregon voters also passed their version of the proposal after the results proved too close to call on Election Day.

Louisiana's voters rejected their own, similar proposal – Amendment 7 – which read, "Do you support an amendment to prohibit the use of involuntary servitude except as it applies to the otherwise lawful administration of criminal justice?"

Voters in Colorado became the first to approve removal of slavery exception language from the state constitution in 2018, followed by Nebraska and Utah two years later.

The movement to end or regulate the use of prison labor has existed for decades, since the time when former Confederate states sought ways to maintain the use of chattel slavery after the Civil War. Southern states used racist laws, referred to as "Black codes," to criminalize, imprison and re-enslave Black Americans over benign behavior.