CLINTON, Md. — Donna Edwards crystallized her case against Chris Van Hollen here one April night by telling a story about a Mississippi Republican and a space center. The Democratic congresswoman was speaking to only a few dozen voters, most of them African American, about a half-hour from the nation’s capital, a low-key event for an otherwise rollicking primary that has drawn national attention.

If Republican Barbara Comstock ever wonders what attacks are coming her way later this year, the Virginia congresswoman can read the entire 535-page opposition research book on her life, which Democrats posted online.