DETROIT — Michigan’s Republican governor announced Friday that Democrat John Conyers’ congressional seat will not be filled until the regularly scheduled November election, leaving it vacant for nearly a year.

Gov. Rick Snyder decided the post will be listed twice on the August primary and November general election ballots. While unlikely, it is possible voters could choose one candidate to fill the vacancy until January 2019 and elect another to a full two-year term after that.

It is unusual for a congressional district to stay vacant for so long, according to a review of vacancies and successors listed on the House website. In the last 20 years, the longest time a seat stayed empty was 10 months — in 2014, when Rep. Melvin Watt of North Carolina left to head the Federal Housing Finance Agency.

Snyder said he opted against having an earlier special election to give potential candidates ample time to decide about running, provide voters in the predominantly Democratic district more options and save money.

The 88-year-old Conyers, who was facing a House Ethics Committee investigation over claims by former staffers, cited health reasons for his resignation Tuesday.

Michael Gilmore, a Detroit attorney who is running for the seat, said not having representation in the House for nearly a year is unfair to residents.

“I think it’s unfortunate that the governor thinks this is what we’re worth,” Gilmore said Friday.

Snyder’s office said it consulted with Wayne County leadership before making a decision. The 13th Congressional District Democratic Party Organization backed the decision, too.

“In order to allow several months for that to take place and to reduce the financial burden on local taxpayers, the primary and general elections will be held when regularly scheduled elections are already occurring,” Snyder said in a statement.

The filing deadline is April 24 for both elections. Whoever wins the special election will serve next November and December, and — if he or she also wins the regular race — will serve a two-year term starting in 2019. A Snyder spokeswoman said it could have cost up to $2 million if the special elections were scheduled on non-regular election dates.