Voters in four states approved ballot measures that will change their state constitutions to prohibit slavery and forcing someone to work against their will as punishment for crime. The initiatives won't force immediate changes in the states' prisons, but they may invite legal challenges over the practice of pressuring prisoners to work under threat of punishment or loss of privileges if they refuse the work. The results were celebrated among anti-slavery advocates, including those pushing to further amend the U.S. Constitution, which prohibits enslavement and forced work except as a form of criminal punishment. Nearly 160 years after enslaved Africans and their descendants were released from bondage through ratification of the 13th Amendment, the slavery exception continues to allow jails and prisons to use inmates for low-cost labor. U.S. Senator Jeff Merkley of Oregon and Representative Nikema Williams of Georgia, both Democrats, reintroduced legislation to revise the 13th Amendment to end the slavery exception. If it wins approval in Congress, the constitutional amendment must be ratified (approved) by three-fourths of the states. After Tuesday's vote, more than a dozen states still have constitutions that include language permitting slavery and forced labor for prisoners. Prison labor is a multibillion-dollar practice. Workers usually make less than $1 per hour, sometimes only pennies. Prisoners who refuse to work can be denied privileges such as phone calls and visits with family.