| Year | Won | Margin | Democratic | Republican | Total |
|---|---|---|---|---|---|
| 2024 | D | 59,942 | 59,257 | 121,347 | |
| 2020 | D | 64,056 | 60,861 | 128,160 | |
| 2016 | D | 55,713 | 55,624 | 120,055 | |
| 2012 | D | 61,732 | 55,138 | 119,397 | |
| 2008 | D | 70,034 | 53,886 | 126,031 | |
| 2004 | R | 59,740 | 60,782 | 121,425 | |
| 2000 | R | 51,981 | 53,816 | 109,297 | |
| 1996 | D | 46,264 | 44,479 | 99,910 | |
| 1992 | D | 48,298 | 42,221 | 112,215 | |
| 1988 | R | 45,280 | 55,699 | 101,550 | |
| 1984 | R | 44,629 | 64,203 | 109,451 | |
| 1980 | R | 32,384 | 48,825 | 105,089 | |
| 1976 | R | 42,399 | 52,736 | 97,387 | |
| 1972 | R | 35,937 | 57,682 | 93,850 | |
| 1968 | R | 36,702 | 47,646 | 90,739 | |
| 1964 | D | 48,834 | 39,920 | 88,754 | |
| 1960 | R | 40,090 | 49,541 | 89,741 | |
| 1956 | R | 29,063 | 48,332 | 77,484 | |
| 1952 | R | 31,409 | 43,468 | 75,004 | |
| 1948 | R | 27,145 | 29,537 | 57,313 | |
| 1944 | R | 27,831 | 30,837 | 58,945 | |
| 1940 | R | 28,061 | 30,683 | 59,127 | |
| 1936 | D | 27,200 | 24,997 | 53,989 | |
| 1932 | R | 17,707 | 26,632 | 46,625 | |
| 1928 | R | 7,684 | 33,258 | 41,372 | |
| 1924 | R | 2,228 | 21,978 | 30,814 | |
| 1920 | R | 3,355 | 19,913 | 25,134 | |
| 1916 | R | 6,198 | 14,893 | 22,882 | |
| 1912 | R | 2,276 | 2,537 | 13,105 | |
| 1908 | R | 2,163 | 8,919 | 12,467 | |
| 1904 | R | 1,177 | 8,143 | 10,887 | |
| 1900 | R | 2,498 | 8,103 | 11,147 | |
| 1896 | R | 2,447 | 8,242 | 10,975 | |
| 1892 | R | 2,634 | 5,854 | 9,366 | |
| 1888 | — | — | — | — | |
| 1884 | — | — | — | — | |
| 1880 | — | — | — | — | |
| 1876 | — | — | — | — |
Home to Rockford, Illinois's third-largest city, Winnebago County shifted from reliably Republican to a genuine battleground as its mid-sized industrial economy restructured and its suburban precincts trended more competitive through the 2010s and 2020s.
The county's recent history is a story of close margins. The Democratic margin reached thirteen points in 2008; the Republican margin reached sixty-six points in 1920. Most other elections have been decided by single-digit points.
Winnebago County's demographics — a population of 283,292, a 66% non-Hispanic-white share, a median household income of $65,837 — 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 DeKalb County and Lehigh County.