The Republican Party had been historically dominant in county-level politics in the 19th and early 20th centuries; prior to the Great Depression, Pittsburgh and Allegheny County had been majority Republican. Since the Great Depression on the state and national levels, the Democratic Party has been dominant in county-level politics. It is by far the most Democratic county in western Pennsylvania. For much of the time between the Great Depression and the turn of the millennium, it was the second-strongest Democratic bastion in Pennsylvania, behind only Philadelphia.
In 2000, Democrat Al Gore won 56% of the vote and Republican George W. Bush won 41%. In 2004, Democrat John Kerry received 57% of the vote and Republican Bush received 41%. In 2006, Democrats Governor Ed Rendell and Senator Bob Casey, Jr. won 59% and 65% of the vote in Allegheny County, respectively. In 2008, Democrat Barack Obama received 57% of the vote, John McCain received 41%, and each of the three state row office winners (Rob McCord for Treasurer, Jack Wagner for Auditor General, and Tom Corbett for Attorney General) also carried Allegheny.
In 2016, despite Donald Trump being the first Republican to carry Pennsylvania since 1988, Hillary Clinton did slightly better than Barack Obama's 2012 vote total while Donald Trump was the worst performing Republican in 20 years. In the 2018 Midterms, Democrats received an even higher percentage of the vote with Tom Wolf and Bob Casey receiving approximately two thirds of the county's vote. This is an improvement over the approximately 55% each person received in the county in their last election in 2014 and 2012 respectively. In 2020, Joe Biden improved upon Clinton's performance, receiving the highest vote percentage for a Democrat since Michael Dukakis in 1988 and the most votes for a Democrat since LGJ in 1964.