| Year | Won | Margin | Democratic | Republican | Total |
|---|---|---|---|---|---|
| 2024 | D | 184,281 | 137,299 | 326,429 | |
| 2020 | D | 182,372 | 128,565 | 314,502 | |
| 2016 | D | 141,682 | 116,114 | 272,998 | |
| 2012 | R | 124,311 | 124,840 | 252,576 | |
| 2008 | D | 137,833 | 114,421 | 254,354 | |
| 2004 | R | 109,708 | 120,036 | 230,823 | |
| 2000 | R | 82,047 | 100,080 | 187,676 | |
| 1996 | R | 64,783 | 77,029 | 158,366 | |
| 1992 | R | 59,643 | 74,002 | 169,208 | |
| 1988 | R | 44,853 | 93,522 | 139,585 | |
| 1984 | R | 38,870 | 92,221 | 131,531 | |
| 1980 | R | 34,307 | 73,046 | 119,896 | |
| 1976 | R | 42,712 | 67,686 | 112,026 | |
| 1972 | R | 31,118 | 72,726 | 106,259 | |
| 1968 | R | 32,606 | 56,073 | 98,051 | |
| 1964 | D | 47,940 | 40,280 | 88,610 | |
| 1960 | R | 30,167 | 53,059 | 83,373 | |
| 1956 | R | 19,957 | 47,225 | 67,232 | |
| 1952 | R | 21,490 | 39,961 | 61,615 | |
| 1948 | R | 14,670 | 29,258 | 44,478 | |
| 1944 | R | 18,548 | 26,655 | 45,411 | |
| 1940 | R | 22,473 | 28,222 | 50,878 | |
| 1936 | R | 26,676 | 29,340 | 56,629 | |
| 1932 | R | 12,040 | 29,425 | 42,517 | |
| 1928 | R | 7,689 | 36,659 | 44,558 | |
| 1924 | R | 5,946 | 22,333 | 29,480 | |
| 1920 | R | 7,004 | 18,129 | 26,060 | |
| 1916 | R | 8,514 | 11,845 | 20,864 | |
| 1912 | D | 6,901 | 5,708 | 19,786 | |
| 1908 | R | 6,555 | 13,118 | 20,476 | |
| 1904 | R | 4,342 | 14,200 | 19,215 | |
| 1900 | R | 6,214 | 13,809 | 20,858 | |
| 1896 | R | 6,058 | 14,232 | 20,990 | |
| 1892 | R | 7,850 | 10,982 | 19,764 | |
| 1888 | — | — | — | — | |
| 1884 | — | — | — | — | |
| 1880 | — | — | — | — | |
| 1876 | — | — | — | — |
Once a reliably Republican suburban stronghold, Chester County has trended Democratic over the past decade as college-educated voters realigned, delivering a 14.5-point Democratic margin in 2024.
The county's recent history is a story of close margins. The Democratic margin reached seventeen points in 2020; the Republican margin reached sixty-five points in 1928. Most other elections have been decided by single-digit points.
Chester County's demographics — a population of 547,840, a 77% non-Hispanic-white share, a median household income of $127,208 — situate the county close to national averages on several dimensions. The county's voting pattern over the last decade is most similar to that of DuPage County and Kane County.
