The Democratic Party historically dominated county-level politics between the New Deal realignment and the turn of the century; however, Westmoreland has trended Republican at the national and statewide levels in the 21st century (in direct lockstep with most other Appalachian counties). In 2000, Republican George W. Bush became the first Republican to carry the county since 1972, and Republicans have carried the county in every election since, increasing the margin of victory in every successive election until 2020. Democratic Governor Ed Rendell lost Westmoreland in both 2002 and 2006. In 2008, Republican Tim Krieger picked up the 57th House district left open by the retirement of Democratic state representative Tom Tangretti. In 2010, both Pat Toomey and Tom Corbett won Westmoreland in their statewide bids. Also, the GOP gained control of two more State House districts, the 54th with Eli Evankovich and the 56th with George Dunbar. In 2011, the Republican Party swept all county row offices. A Democratic resurgence in 2015 gave that party a majority of the county commissioners. However, in the 2019 elections, Democratic elected officials lost that majority and carried only one row office.

As of 2020, the only majority-Dem cities within the county are Arnold and Monessen.