In 2018, 88 regular state legislative races were decided by margins under 0.5%, including 16 races decided by 10 or fewer votes and two which were decided by a single vote. Eighteen of these races took place in New Hampshire, three times as many as in any other state.
Regular state legislative elections for 6,073 seats in 87 of the nation’s 99 state legislative chambers took place in 2018.
Across all 6,073 seats, the average margin of victory—defined as the difference between the vote share of the winning candidate and the runner-up—was 25.8%. In comparison, the average margin of victory across the 467 seats in the U.S. Congress that were up for election that year was 29.2% (16.8% across 33 U.S. Senate seats and 30.2% across 434 U.S. House seats).
Across all chambers, the smallest average margin of victory was 7.7% in the South Dakota House of Representatives and the largest was 51.6% in the Tennessee State Senate. The average nationwide margin of victory was higher for Democrats (26.8%) than for Republicans (22.3%).
In 2020, state legislative seats in 4,798 districts which held elections in 2018 will be up for election again. Republicans won seats in 2,454 of those districts in 2018, while Democrats won seats in 2,375. The 2018 elections in these districts were decided by a smaller margin than the overall average (24.9% compared to 25.8% overall). The average margin in the districts where Republicans won was 24.5%, while the average margin in districts where Democrats won was 27.8%.
Of the chambers holding elections again in 2020, the South Dakota House of Representatives had the smallest average margin of victory for Democrats at 1.9%, while the Vermont State Senate had the smallest average margin of victory for Republicans at 4.5%. The largest average margin of victory for Democrats was 55.2% in the New York State Assembly while the largest for Republicans was 40.2% in the Tennessee House of Representatives.