Maryland’s 6th Congressional District, with the state’s only competitive House seat in November’s general election, stretches from Gaithersburg and Germantown in liberal Montgomery County to Frederick, Hagerstown and Cumberland in the conservative western panhandle. Democrat John Delaney flipped the seat in 2012 by defeating longtime Republican incumbent Roscoe Bartlett. Rep. David Trone (D) won in 2018 when Mr. Delaney stepped aside for a presidential bid. With Mr. Trone running for the U.S. Senate, April McClain Delaney — the former congressman’s wife — is the best choice for Democrats in the May 14 primary.