Ms. Norton, the civil rights leader and law professor-turned-congresswoman who is the oldest person serving in the House, won election in 1991 and came to be known as D.C.’s “warrior on the Hill,” as she fought doggedly for statehood and for the District’s prerogatives. But in recent years, her once forceful voice has become muted amid increasingly concerning signs of mental and physical decline, leaving her vastly diminished and struggling to fulfill her congressional duties.