Ada County is located in the southwestern part of Idaho, United States.

The last victory in a presidential election by a Democrat in Ada County was by Franklin D. Roosevelt in 1936.

Barry Goldwater carried it by 13 points, a major reason why Idaho was Johnson's closest state.

[12] In 2008 the presidential election in Ada County was more competitive than in previous years; John McCain defeated Barack Obama by six percentage points.

Obama became the first Democrat to garner as much as 40 percent of the county's vote since Lyndon B. Johnson.

In 2016, Donald Trump won the county by only a plurality due to high third party performance.

In 2020, whereas the state of Idaho voted very strongly for Trump, he carried Ada County by a slim majority, only beating Joe Biden by around 3 percentage points.

The election was close due to Biden's strength in Boise's downtown urban core, which gave him 59.2% of the vote to Trump's 37.4%.

This was the best performance for a Republican presidential candidate in Ada county since George W. Bush in 2004.

Democratic gubernatorial nominee Jerry Brady carried the county in his 2002 and 2006 races, despite losing statewide in both contests.

[21] Each party held all of their respective legislative seats in the 2008 elections, but Republicans won two competitive county commission races.

Other elected officials include clerk, treasurer, sheriff, assessor, coroner, and prosecutor.

[25] The Idaho Department of Correction operates the South Boise Prison Complex,[35] a correctional facility located in unincorporated Ada County, south of the Boise Airport and east of Kuna.

[36] School districts include:[37] Residents are in the area (and the taxation zone) for College of Western Idaho.