The 12th Pennsylvania congressional district is located in southwestern Pennsylvania. It is a heavily Gerrymandered district. It consists of all of Greene County, and parts of Allegheny, Armstrong, Cambria, Fayette, Indiana, Somerset, Washington, and Westmoreland Counties.

The district was drawn specifically for Murtha, including many heavily Democratic regions, while leaving more right-leaning Pittsburgh suburban regions to the 4th or 18th district, and rural conservative regions to the 3rd or 9th district.[citation needed]

The 12th includes all of Greene County, a highly rural region that still has a traditionally Democratic influence due to its labor leanings. In Washington county, the city of Washington, a large and Democratic edge suburb of Pittsburgh is a part of the 12th, as well as the eastern portion of the county, which includes the very Democratic former steel megacenter, the Monongahela Valley region. However, more rural western Washington County and the suburban northern portion of the county (with towns like McDonald and Canonsburg) belongs to the 18th. The western portion of Fayette County, including the city of Uniontown, a labor Democratic stronghold is part of this district, while the rural mountainous eastern portion is a part of the 9th.

The 12th District continues eastward, including southeastern and northeastern parts of Westmoreland County, including the labor Democratic city of Latrobe, while leaving the suburban western part of the county (with towns such as Murrysville) and the generally left-leaning city of Greensburg in the 18th. Portions of Somerset and Cambria Counties are also included in this district. This strip, the part of a large coal area with Democratic leanings, is Murtha's main base. It includes his home city of Johnstown. The 12th also contains a part of Indiana County, mainly the college town of Indiana.

The 12th completes its wrap around the metro Pittsburgh region by ending in the northeastern corner of the city's suburbs, containing middle class regions such as Lower Burrell and the working class suburb of New Kensington. A portion of Armstrong County is also included in the district, including several industrial suburbs such as Freeport and Apollo.