Voters will choose state senators in 12 districts Tuesday, including in Edmond, Norman, Lawton, and for a new Senate seat created by redistricting in south-central Oklahoma.
The session that ended in May had 32 Republicans and 16 Democrats in the Senate.
Republicans won two seats in the August runoff election that previously had been held by Democrats.
Both parties are watching the five races that don't have incumbents while hoping some of their challengers can unseat the seven senators that are seeking re-election.
Republicans are rooting for upsets in the historically Democratic seats like District 7 in Haskell, Latimer, Pittsburg counties and parts of Hughes and Okfuskee counties. Democrats are hoping to claim Senate District 15 in Norman, a seat held for 12 years by Republican Jonathan Nichols.
The district runs from Norman in Cleveland County to Moore in southern Oklahoma County. The candidates are Democrat Claudia Griffith, a registered nurse, and Republican Rob Standridge, a pharmacist and small-business owner.