| Year | Won | Margin | Democratic | Republican | Total |
|---|---|---|---|---|---|
| 2024 | D | 192,641 | 92,427 | 297,369 | |
| 2020 | D | 209,940 | 99,073 | 320,326 | |
| 2016 | D | 153,251 | 83,197 | 269,232 | |
| 2012 | D | 135,291 | 93,974 | 237,023 | |
| 2008 | D | 141,544 | 89,185 | 236,632 | |
| 2004 | D | 121,140 | 107,223 | 231,308 | |
| 2000 | D | 90,662 | 86,091 | 185,974 | |
| 1996 | D | 76,619 | 65,221 | 159,755 | |
| 1992 | D | 67,528 | 57,146 | 167,195 | |
| 1988 | R | 59,837 | 67,018 | 129,211 | |
| 1984 | R | 44,602 | 75,877 | 120,896 | |
| 1980 | R | 37,915 | 57,165 | 111,355 | |
| 1976 | R | 34,847 | 52,376 | 90,611 | |
| 1972 | R | 27,890 | 43,958 | 75,238 | |
| 1968 | R | 22,943 | 34,105 | 59,842 | |
| 1964 | D | 29,081 | 20,813 | 50,181 | |
| 1960 | R | 17,736 | 25,415 | 43,186 | |
| 1956 | R | 14,027 | 22,001 | 36,028 | |
| 1952 | R | 11,191 | 20,250 | 31,584 | |
| 1948 | R | 9,424 | 11,455 | 21,589 | |
| 1944 | R | 9,110 | 9,362 | 18,677 | |
| 1940 | D | 8,626 | 8,367 | 17,103 | |
| 1936 | D | 8,641 | 4,148 | 13,612 | |
| 1932 | D | 6,824 | 4,201 | 11,573 | |
| 1928 | R | 3,544 | 6,162 | 9,879 | |
| 1924 | R | 2,103 | 4,203 | 9,141 | |
| 1920 | R | 2,262 | 4,947 | 7,641 | |
| 1916 | R | 3,363 | 4,888 | 8,703 | |
| 1912 | D | 1,429 | 1,261 | 4,659 | |
| 1908 | R | 1,153 | 2,319 | 3,743 | |
| 1904 | R | 492 | 2,296 | 3,137 | |
| 1900 | R | 1,114 | 1,655 | 2,948 | |
| 1896 | R | 1,566 | 2,082 | 3,708 | |
| 1892 | R | 293 | 1,587 | 2,979 | |
| 1888 | — | — | — | — | |
| 1884 | — | — | — | — | |
| 1880 | — | — | — | — | |
| 1876 | — | — | — | — |
Washington County has shifted from competitive terrain to a reliable Democratic stronghold over two decades, driven by rapid population growth in the Hillsboro–Beaverton tech corridor and an increasingly diverse, college-educated electorate.
The Democratic margin in Washington County has been steady. It reached its modern peak at thirty-five points in 2020; the 2024 margin was thirty-four points, still in line with the county's long pattern.
Washington County's political identity is inseparable from its demographic profile: a 64% non-Hispanic-white share, a median household income of $107,772, and the full diversity of a major metropolitan center. The county's voting pattern resembles other major urban centers most closely — Howard County and Santa Barbara County.
