If the O's were to ever leave Bmore (doubtful), the Nats would only gain marginally in attendance. Unlike the MANY DC area fans who flocked to the O's in the absence of baseball in DC, there is a visceral, longstanding hatred of all things Washington in Baltimore.
You could probably count the Bmore football fans who "adopted" the Redskins during their football "drought" on two hands. They became Steeler fans before Redskins fans which, given the current rivalry with Steeltown, tells you a lot.