IOWA CITY, Iowa (AP) — A Republican candidate seeking to flip a Democratic-held Iowa congressional seat pulled ahead by 40 votes Tuesday after a county said that it failed to report accurate results from a small town on election night.

The dramatic turn was the latest in the seesawing race between Republican Mariannette Miller-Meeks and Democrat Rita Hart, and it may not be the last.

The 24 counties in the southeastern Iowa district were certifying their votes Tuesday after completing their official canvasses, which included some additional provisional ballots or absentee ballots that arrived by Monday at noon.

The race is considered among the closest in the nation and could remain unsettled for weeks. The Associated Press has not determined a winner.

Lucas County Auditor Julie Masters said she discovered Monday that the results for a precinct in the town of Russell, population 500, had not been reported accurately after polls closed. She said test results in the software used for reading machine cartridges were reported instead of the actual results, which added 217 votes for Miller-Meeks and 54 for Hart.

“The machine did not lead us down the wrong path,” she said at a news conference in Chariton. “Through human error, we didn’t get all the results reported correctly.”

The change erased Hart's slim lead. By Tuesday afternoon, Miller-Meeks was leading by 40 votes out of more than 394,000 cast as other minor adjustments from counties trickled in as they certified their totals.

Iowa Secretary of State Paul Pate ordered Lucas County to conduct a machine recount of all of its votes, and a hand recount for the precinct in question. He said that process would begin Thursday.

“We want to make sure we get it right,” he said.

Hart’s campaign expressed anger at the situation in Lucas County, where the Board of Supervisors quickly certified the corrected results on Tuesday morning.

A Hart campaign lawyer accused Pate, a Republican, of pressuring reluctant supervisors to take that step only hours after Pate's office notified the campaign of the error late Monday.

“It is outrageous that Lucas would identify a race defining change in the middle of the night the night before the county canvass, six days after the election, and then rush to certify results just hours later before accuracy has been ensured,” campaign manager Zachary Meunier said.

Pate said the county's certified totals would be amended if the recounts change them.

His spokesman, Kevin Hall, said the county was required to complete the official canvass by the end of the day Tuesday and “never once talked to anyone in our office about delaying it.”

The race appears headed for a much wider recount in any event. The winner must be certified by Nov. 30 by a panel of top state officials.

Either candidate can ask for a recount through Friday for any or all of the district’s counties. That process would be done at the state’s expense given the razor-thin margin of votes separating the two.

The seat has been held for seven terms by Democratic U.S. Rep. Dave Loebsack, whose retirement created a vacancy in a district that Donald Trump narrowly won in 2016.

A victory by Miller-Meeks would flip a second Iowa House seat to the GOP, after Republican Ashley Hinson unseated first-term Democratic Rep. Abby Finkenauer last week. Miller-Meeks is making her fourth run for Congress.

Hart, a former state senator and 2018 candidate for lieutenant governor, trailed by less than 300 votes out of more than 393,000 cast in unofficial results after polls closed on election night. She did not concede to Miller-Meeks, a state senator who expressed confidence her lead would stand.

Hart pulled ahead by less than 200 votes days later after officials discovered and corrected an overcount in one precinct in Jasper County that had mistakenly inflated Miller-Meeks’ totals by hundreds of votes. A machine recount and then a daylong hand recount Monday of that precinct’s ballots validated the correction.

But Hart’s lead quickly vanished later Monday when officials fixed the error in Lucas County.