Judith Pearson’s home in rural Minnesota is stocked with guns for hunting and recreational use, but that didn’t stop her from venting her frustration with the National Rifle Association after the deadly school shooting in Parkland, Fla.

Ms. Pearson, a retired school principal, was irked by the group’s repeated efforts to preserve civilian access to semiautomatic rifles, like the AR-15 the gunman used to kill 17 students and school staff members. “BoycottNRA,” she wrote to her handful of Twitter followers on the day of the attack, becoming one of the first participants in what became a sprawling campaign to force corporate America to dissociate itself from the gun lobby.

Across the country, in Los Angeles, Laura Mannino had a similar thought.

Drain the NRA, an advocacy group she co-founded in October after a deadly shooting in Las Vegas, developed a public spreadsheet of businesses associated with the gun group. After the attack in Florida, Ms. Mannino helped organize a rally against gun violence in downtown Los Angeles that drew hundreds of people.

“Everyone started, in parallel ways, but in tandem, to go after these companies,” she said.

Since the Parkland massacre, a range of people and coalitions have converged on the nationwide battleground over gun control. That includes individuals trying to make a statement, activist organizations sensing a shift in sentiment, informal parents’ clubs, marketing executives, niche media groups, celebrities and shooting survivors.