After the Parkland shooting, Lindsey Donovan, an Army veteran in Georgia who is a member of Moms Demand Action for Gun Sense in America, said she began getting calls and inquiries from distant corners of that state, including gun shop owners in tiny towns “wanting to talk about what it is we are about.” Conversations about gun restrictions with friends in the military, she said, seemed to grow more searching and less combative.

“They are not shutting me down anymore,” Ms. Donovan said.

Before Parkland, her organization in Georgia had five local groups. Now there are 12. Around 1,800 people showed up to the organization’s annual advocacy day in Atlanta in late February, up from 150 last year. And local chapter meetings used to draw around 30 people on any given night. Now it’s often more than 100 showing up.