| Year | Won | Margin | Democratic | Republican | Total |
|---|---|---|---|---|---|
| 2024 | D | 23,648 | 22,716 | 47,401 | |
| 2020 | D | 24,643 | 21,905 | 47,989 | |
| 2016 | D | 20,466 | 19,091 | 43,600 | |
| 2012 | D | 21,207 | 18,934 | 41,241 | |
| 2008 | D | 25,784 | 18,266 | 44,818 | |
| 2004 | R | 19,263 | 21,095 | 40,768 | |
| 2000 | R | 14,798 | 17,139 | 33,233 | |
| 1996 | D | 12,715 | 12,380 | 28,527 | |
| 1992 | D | 13,744 | 12,655 | 34,232 | |
| 1988 | R | 11,811 | 17,182 | 29,190 | |
| 1984 | R | 10,942 | 20,294 | 31,465 | |
| 1980 | R | 8,913 | 16,370 | 30,365 | |
| 1976 | R | 11,535 | 18,193 | 30,728 | |
| 1972 | R | 12,375 | 18,910 | 31,384 | |
| 1968 | R | 6,974 | 14,535 | 22,999 | |
| 1964 | R | 10,257 | 11,791 | 22,049 | |
| 1960 | R | 6,783 | 15,586 | 22,388 | |
| 1956 | R | 4,826 | 15,078 | 19,929 | |
| 1952 | R | 5,110 | 14,807 | 19,947 | |
| 1948 | R | 5,082 | 11,380 | 16,567 | |
| 1944 | R | 6,004 | 12,157 | 18,210 | |
| 1940 | R | 6,989 | 12,577 | 19,668 | |
| 1936 | R | 7,899 | 9,826 | 18,275 | |
| 1932 | R | 6,923 | 9,356 | 16,594 | |
| 1928 | R | 3,940 | 11,501 | 15,503 | |
| 1924 | R | 1,540 | 10,500 | 13,744 | |
| 1920 | R | 1,700 | 10,374 | 12,361 | |
| 1916 | R | 3,386 | 9,764 | 13,697 | |
| 1912 | R | 1,568 | 1,776 | 7,314 | |
| 1908 | R | 1,732 | 5,866 | 8,091 | |
| 1904 | R | 1,137 | 5,957 | 7,693 | |
| 1900 | R | 1,881 | 5,923 | 8,110 | |
| 1896 | R | 1,881 | 5,598 | 7,734 | |
| 1892 | R | 1,926 | 3,789 | 6,240 | |
| 1888 | — | — | — | — | |
| 1884 | — | — | — | — | |
| 1880 | — | — | — | — | |
| 1876 | — | — | — | — |
Home to Northern Illinois University, DeKalb County shifted from reliably Republican to a near-even split over the past two decades, with the 2024 presidential race decided by roughly 2 points — a margin driven largely by turnout patterns around Dekalb's student and exurban populations.
The county's recent history is a story of close margins. The Democratic margin reached seventeen points in 2008; the Republican margin reached seventy points in 1920. Most other elections have been decided by single-digit points.
DeKalb County's demographics — a population of 100,703, a 74% non-Hispanic-white share, a median household income of $70,724 — 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 Centre County and Will County.