In Washington county, the city of Washington and eastern portions of the county, a large and Democratic edge suburb of Pittsburgh, was a part of the 12th. Most of the Monongahela Valley region, a very Democratic area once an important steel-making area, was also part of the 12th.
Washington County, specifically the city of Washington and its eastern suburbs, were located within the 12th congressional district.  The Monongahela Valley, a traditionally Democratic region known for its steel industry, was also part of this district.