Voters in four states have approved ballot measures that will change their state constitutions to prohibit slavery and involuntary servitude as punishment for crime. The measures could curtail the use of prison labor in Alabama, Oregon, Tennessee and Vermont. In Louisiana, a former slave-holding state, lawmakers trying to pass a similar measure ended up torpedoing it over ambiguous language. Anti-slavery advocates celebrated the results as a signal for what is possible in the effort to end the slavery exception in the U.S. Constitution.