The measure known as Question 2 was defeated 51-49 percent. It would have placed Massachusetts alongside Oregon and Washington state in legalizing the controversial practice.
Catholic leaders helped lead the opposition to Question 2.
"It is my hope and prayer that the defeat of Question 2 will help all people to understand that for our brothers and sisters confronted with terminal illness we can do better than offering them the means to end their lives," Boston Archbishop Sean O'Malley said in a statement.
Elsewhere, social conservatives had a mixture of wins and losses:
-- In Alabama, Roy Moore won the race for chief justice of the Alabama Supreme Court with 52 percent of the vote, meaning he will return to the position he once held. Moore rose to prominence by placing a Ten Commandments monument in the lobby of the Alabama Judicial Building. He was removed from office by a judicial panel after he refused to obey an order to remove the monument. He has said he will not return the monument to the building.
-- In Montana, voters passed a pro-life measure (Referendum 120) requiring that parents be notified when girls under the age of 16 try to obtain an abortion. It passed with 66 percent of the vote.
-- In Iowa, voters retained state Supreme Court Justice David Wiggins, who was up for retention three years after the court legalized gay marriage in a unanimous 7-0 vote. Wiggins won retention, 55-45 percent.
-- In Maryland, an initiative passed (52-48 percent) that will allow a Las Vegas-style casino in Prince George's County and table-type gambling at the five casinos already in existence.
-- In Rhode Island, two gambling measures won that will permit slot parlors in two specific localities to add table games. One won with 71 percent, the other 67 percent.
Compiled by Michael Foust, associate editor of Baptist Press. Get Baptist Press headlines and breaking news on Twitter (@BaptistPress), Facebook (Facebook.com/BaptistPress ) and in your email ( baptistpress.com/SubscribeBP.asp).
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