After the massacre of schoolchildren at Sandy Hook Elementary in Newtown, Conn., in 2012, the gun-control movement was small and badly outspent by the National Rifle Association. Parents seeking an outlet for their grief and rage congregated on Facebook, where they formed their own group, Moms Demand Action, to push for stricter gun laws.By far the most significant and best-known donor in the years since has been Michael Bloomberg, the billionaire and former New York City mayor. In 2013, his mayors’ initiative merged with Moms Demand Action to create Everytown for Gun Safety, the closest thing that the gun-control movement has to a counterweight to the N.R.A. That year the group spent $36.5 million, compared with $4.7 million the year …
