Lucy McBath said she was anxious as she sat outside a Caribbean restaurant in Sandy Springs, Ga., on Tuesday night, surrounded by her supporters as she waited to learn whether her nascent political career would continue.

By 11 p.m., Ms. McBath, 58, who became an anti-gun violence activist after her son was fatally shot in 2012, had her answer: She would become the Democratic nominee for the Sixth Congressional District’s seat after winning a runoff election against Kevin Abel, 53.

“I couldn’t believe it,” Ms. McBath said in an interview on Wednesday. “The first thing I said was, ‘Thank you, Lord.’”

Ms. McBath, who received 53.7 percent of the vote against Mr. Abel, a businessman and first-time candidate, will now face Representative Karen Handel in the general election in November. This year’s midterm elections have a surge of progressive female candidates, and Ms. McBath will stand in stark contrast to Ms. Handel, 56, who has been endorsed by the National Rifle Association.