Nashua Mayor Donalee Lozeau quit New York City Mayor Michael Bloomberg’s anti-gun group Mayors Against Illegal Guns after it exposed its true mission: to pursue strict new gun-control laws. She has had plenty of company. A review of the group’s membership by news and entertainment website Buzzfeed.com found that it has lost more than 50 members who have quit, lost re-election or had their terms expire. Lozeau quit after she saw the group’s first ad attacking U.S. Sen. Kelly Ayotte for voting against a gun-control bill. “I said, ‘Wait a minute, I don’t want to be part of something like...