Representative Mikie Sherrill, a New Jersey Democrat, said a firm once owned by her Republican opponent, Jack Ciattarelli, spread misinformation about opioids. His campaign has threatened to sue her for defamation.

© Kriston Jae Bethel and Rachel Wisniewski for The New York Times

Viral videos showing caustic behavior have blunted her momentum in the California governor’s race. Other campaigns are scrambling to take advantage.

© Ariana Drehsler for The New York Times