| Year | Won | Margin | Democratic | Republican | Total |
|---|---|---|---|---|---|
| 2024 | R | 4,082 | 5,605 | 9,803 | |
| 2020 | R | 4,720 | 5,492 | 10,321 | |
| 2016 | R | 4,400 | 4,889 | 9,485 | |
| 2012 | D | 5,188 | 4,659 | 9,923 | |
| 2008 | D | 4,931 | 4,769 | 9,798 | |
| 2004 | R | 3,982 | 4,606 | 8,685 | |
| 2000 | R | 3,661 | 4,521 | 8,276 | |
| 1996 | R | 3,620 | 3,808 | 7,774 | |
| 1992 | R | 3,344 | 4,026 | 8,177 | |
| 1988 | R | 2,564 | 4,467 | 7,075 | |
| 1984 | R | 2,811 | 4,346 | 7,189 | |
| 1980 | D | 3,399 | 3,228 | 6,705 | |
| 1976 | D | 4,083 | 2,569 | 6,652 | |
| 1972 | R | 1,560 | 3,955 | 5,515 | |
| 1968 | R | 1,716 | 1,849 | 5,916 | |
| 1964 | R | 1,382 | 3,670 | 5,052 | |
| 1960 | R | 1,331 | 1,842 | 3,173 | |
| 1956 | D | 1,914 | 520 | 3,009 | |
| 1952 | D | 1,598 | 657 | 2,255 | |
| 1948 | D | 115 | 28 | 2,064 | |
| 1944 | D | 1,482 | 8 | 1,506 | |
| 1940 | D | 1,845 | 13 | 1,858 | |
| 1936 | D | 2,157 | 0 | 2,159 | |
| 1932 | D | 1,877 | 0 | 1,892 | |
| 1928 | D | 1,028 | 34 | 1,062 | |
| 1924 | D | 847 | 23 | 872 | |
| 1920 | D | 721 | 25 | 746 | |
| 1916 | D | 1,454 | 21 | 1,476 | |
| 1912 | D | 1,139 | 0 | 1,159 | |
| 1908 | D | 1,407 | 88 | 1,495 | |
| 1904 | D | 1,401 | 35 | 1,436 | |
| 1900 | D | 1,356 | 57 | 1,413 | |
| 1896 | — | — | — | — | |
| 1892 | D | 2,137 | 549 | 2,726 | |
| 1888 | — | — | — | — | |
| 1884 | — | — | — | — | |
| 1880 | — | — | — | — | |
| 1876 | — | — | — | — |
Barnwell's electorate skews older and rural, with a substantial Black population concentrated in the county seat that consistently offsets some of the countywide Republican lean, keeping margins closer in local races than federal ones.
The Democratic margin in Barnwell County has rarely exceeded 100 points in modern history; the Republican margin has rarely exceeded forty-five points. 2024 delivered the county to the Republican candidate by sixteen points.
Barnwell County's demographics resemble the country more than they resemble most counties. A 52% non-Hispanic-white share, a 25% poverty rate, and a median household income of $46,626 — all within the broad national range. The county's voting pattern over the last decade is most similar to that of Porter County and Berks County.