The Supreme Court of Pennsylvania redrew this and other state congressional districts in February 2018 after ruling the previous map unconstitutional due to partisan gerrymandering.

After the 2000 census, the Republican-controlled state legislature radically altered the 12th to get more Republicans elected from traditionally heavily Democratic southwestern Pennsylvania.

This led to criticism that the 12th was a gerrymander intended to pack as many of southwestern Pennsylvania's heavily Democratic areas as possible into just two districts—the 12th and the Pittsburgh-based 14th.

The 12th included all of Greene County, a highly rural region that still has a traditionally Democratic influence due to its labor leanings.

The western portion of Fayette County, including the city of Uniontown, a labor Democratic stronghold, was part of this district.

Pennsylvania governor Ed Rendell scheduled a special election for May 18, 2010, following the death of Representative John Murtha.

On March 8, 2010, the Pennsylvania Democratic Party's Executive Committee nominated Mark Critz, Murtha's former district director.

Mark Critz was re-elected in the regularly scheduled 2010 election, again beating Republican Tim Burns (this time with 51% of the vote against 49%).

Mark Critz ran for re-election to a second full term in the 2012 election but was defeated by Republican challenger Keith Rothfus.