| Year | Won | Margin | Democratic | Republican | Total |
|---|---|---|---|---|---|
| 2024 | R | 20,574 | 28,771 | 50,146 | |
| 2020 | R | 19,904 | 27,208 | 47,979 | |
| 2016 | R | 16,569 | 23,417 | 43,109 | |
| 2012 | R | 20,158 | 23,517 | 44,281 | |
| 2008 | D | 21,448 | 21,096 | 43,166 | |
| 2004 | R | 17,925 | 23,776 | 42,115 | |
| 2000 | R | 15,203 | 19,204 | 36,099 | |
| 1996 | D | 13,188 | 12,681 | 29,774 | |
| 1992 | R | 11,593 | 13,072 | 32,802 | |
| 1988 | R | 11,816 | 14,309 | 26,342 | |
| 1984 | R | 10,788 | 17,780 | 28,784 | |
| 1980 | R | 11,335 | 16,174 | 30,000 | |
| 1976 | R | 12,577 | 15,528 | 28,812 | |
| 1972 | R | 9,303 | 14,621 | 24,587 | |
| 1968 | R | 8,716 | 12,478 | 22,698 | |
| 1964 | D | 13,295 | 8,741 | 22,084 | |
| 1960 | R | 8,757 | 14,133 | 22,929 | |
| 1956 | R | 6,452 | 13,357 | 19,931 | |
| 1952 | R | 6,827 | 13,884 | 20,743 | |
| 1948 | R | 7,256 | 8,244 | 15,728 | |
| 1944 | R | 6,988 | 10,245 | 17,317 | |
| 1940 | R | 7,842 | 10,178 | 18,169 | |
| 1936 | D | 11,144 | 5,599 | 17,324 | |
| 1932 | D | 11,230 | 5,062 | 16,448 | |
| 1928 | R | 6,305 | 8,612 | 15,033 | |
| 1924 | R | 1,374 | 4,250 | 13,611 | |
| 1920 | R | 1,844 | 8,865 | 11,029 | |
| 1916 | R | 3,645 | 3,785 | 7,636 | |
| 1912 | D | 4,381 | 1,926 | 7,021 | |
| 1908 | D | 4,492 | 3,207 | 7,932 | |
| 1904 | D | 3,778 | 3,669 | 7,696 | |
| 1900 | D | 4,127 | 3,729 | 8,063 | |
| 1896 | R | 3,504 | 4,344 | 8,257 | |
| 1892 | D | 4,661 | 2,679 | 7,575 | |
| 1888 | — | — | — | — | |
| 1884 | — | — | — | — | |
| 1880 | — | — | — | — | |
| 1876 | — | — | — | — |
Jefferson County sits in the Glacial Drumlin region southwest of Milwaukee, where a manufacturing and agricultural economy anchors a largely white, working-class electorate. Its 2024 margin of R+16.4 reflects a long drift from the competitive swings it recorded in the early 2000s.
The Republican margin in Jefferson County reached its widest at sixty-four points in 1920. The margin in 2024 was sixteen points, in line with the county's deep historical pattern.
Jefferson County's loyalty is rooted in its place. Median household income of $83,750, a 89% non-Hispanic-white share, and a population of 86,003 together describe a community whose political habits are deeply settled. The county's voting pattern over the last decade is most similar to that of Calumet County and Dubois County.