Voters in southwest Virginia will cast ballots Tuesday to determine the balance of power in the state Senate.

They will participate in a special election to fill the 38th District seat of former state Sen. Phillip Puckett, D-Russell County.

Voters will decide if Republicans maintain their slight edge in the Senate or Democrats regain control.

When Mr. Puckett abruptly resigned in June, it gave the GOP a 20-19 majority.

His resignation came just days before the General Assembly voted to pass the budget without Medicaid expansion, with the GOP-controlled Senate’s backing.

The Republicans’ margin grew to 20-18 after former Sen. Henry L. Marsh III of Richmond resigned last month to take a job with the state ABC Board.

If Democrat Mike Hymes of Tazewell, a member of the county’s board of supervisors, wins Mr. Puckett’s former seat, that should assure Democrats regain control of the Senate.

Mr. Hymes would join the winner of November’s election to fill Mr. Marsh’s former 16th Senate District seat to virtually assure Democrats 20 Senate votes, with Democratic Lt. Gov. Ralph Northam on board to cast tiebreaking votes.

The other two candidates to replace Mr. Puckett: Ben Chafin, R-Hansonville, currently a junior member of the House of Delegates; and Rick Mullins, an independent candidate who is waging his first campaign.