Shannon Watts was a consultant and stay-at-home mother of five in Zionsville, Ind., with maybe 75 Facebook friends when a gunman walked into the Sandy Hook Elementary School in Connecticut in December 2012 and fatally shot 20 children. Ms. Watts sat down at her kitchen table trying to find something similar to Mothers Against Drunk Driving, i.e. a nonpartisan, mom-led advocacy group to protect kids from gun violence. She didn’t find one, so instead wrote a screed on Facebook that eventually became a group called Moms Demand Action for Gun Sense in America.

Today, Moms Demand Action is among the most prominent gun-control organizations in the country, a nonprofit with nearly six million supporters, more than the N.R.A. Ms. Watts now crisscrosses the country to meet with allies, organize volunteers and pressure elected officials. Those efforts have led to gun safety legislation passed in 20 states, with nine of those bills signed into law by Republican governors.

Ms. Watts, who is 48 and living in Colorado, had once worked in corporate communications and had held a nonpolitical job in state government, so she wasn’t exactly a novice in how to shape an effective narrative. If the gun lobby made gun owners fear that tougher laws would mean losing their rifles, she figured she could motivate mothers — of all political leanings — that inaction could mean the worst fear imaginable: losing a child.

Ms. Watts, whose book “Fight Like a Mother” was just released, and her “army of angry moms” have successfully pushed Starbucks, Dick’s Sporting Goods, Panera Bread and other businesses to ban firearms. The efforts have made her a favorite target of pro-gun-rights groups and a symbol of what many Americans see as an infringement on the Second Amendment. She receives so many threats that she often travels with a bodyguard — one who doesn’t carry a gun.