Representative Carolyn McCarthy hopes she can quit Washington for good before another group of families left grieving by gun violence arrives at her office. She suspects otherwise, since she’s got another 10 months to go before finishing out her 18-year career at the end of the year, and she knows better than anyone in Congress the unyielding rhythm of the nation’s gun trauma.

Twenty years ago, Ms. McCarthy was a nurse in an intensive care unit in the Long Island suburbs when her life was shattered by a gunman who rampaged aboard an evening commuter train and murdered her husband, Dennis, and five other passengers. Her 26-year-old son, Kevin, was left severely wounded in the attack. Groping for stability, Ms. McCarthy decided to leave nursing and run for Congress on the strength of what she called an irresistible “passion to reduce gun violence.”

That passion has been severely tested. The issue of gun safety waxed briefly, then waned depressingly across the years as the gun lobby tightened its grip on Capitol Hill. And always, newly grieving families kept showing up at her Washington office. “The hardest thing was meeting the parents,” said Ms. McCarthy. “I was basically the only person that kept talking about gun violence over all those years. Eventually, the newspapers and TV would call me up and ask, ‘What are you going to do about it?’ ”

This grim routine was broken up by her immersion in other important issues like education and financial regulation. Her office displays eight of her enacted bills on issues like retirees’ rights and childhood hunger signed by three presidents. But the flow of victimized families remained steady as they sought someone in Washington who could truly empathize, and not slip timorously past the gun issue with press releases of condolence.