Forty contested primaries took place for the Ohio state legislature on April 28, 2020. There were nine primaries in the state Senate and 31 primaries in the state House. By partisan affiliation, 17 were Democratic primaries and 23 were Republican primaries.

This was a 35.5 percent decrease in the number of primaries from 2018 (62) and the fewest contested primaries in the state since 2012 (35).

Rep. Jeffrey Todd Smith (R), House District 43, was the only incumbent defeated in a primary in Ohio this year. He filed to run for re-election in December 2019, but announced in January 2020 that he would withdraw from the race, though his name remained on the primary ballot. One incumbent lost in a primary election in both the 2018 and 2016 elections.

There was a 22 percent drop in the number of major party candidates from 2018, one of the largest decreases in states whose filing deadlines have passed. In total, 265 candidates—123 Democrats and 142 Republicans—ran this year compared to 341 in 2018. Only Tennessee (-25%) and Oklahoma (-42%) have seen larger percentage decreases in major party candidates.

The number of races without incumbents in each chamber was about average compared to the past decade—five Senate races (31%) and 19 House races (19%) did not feature an incumbent, meaning the races were for open seats. These numbers are down from the decade-high numbers in 2018 when 10 Senate seats (59%) and 32 House seats (32%) were open.

Of the 16 seats up in the Senate, 11 will have an incumbent running in the general election. In the House, of the 99 seats up for election, 78 are set to feature an incumbent.

Heading into the general election, Republicans hold a 24-9 supermajority in the Senate and a 60-38 supermajority in the House. In 2018, Mike DeWine (R) was elected governor, making Ohio one of 21 Republican state government trifectas. General election winners will be responsible for redrawing district lines after the 2020 decennial census. In Ohio, the General Assembly is responsible for redistricting congressional district lines. State legislative district lines will be set by a bipartisan, seven-member state legislative redistricting commission after voters passed a constitutional amendment in 2015, 71-29%.