With just 1,600 votes between the two leading candidates, Louisiana's fifth congressional district won't learn the outcome of its election until next month.
The race between Democratic state house representative Patricia Moore and Republican state house representative Gabe Firment is predicted to advance to a 12 December runoff election, according to the Associated Press.
With 99 per cent of votes counted, Moore has 17.9 per cent of the vote while Firment has 17.2 per cent of the vote. The rest of the vote is split between seven other candidates.
Louisiana is one of seven states that require candidates to win a certain share of the vote to be elected.
Candidates must receive at least 50 per cent of the vote to win outright; if no one does, the two frontrunners advance to a runoff.
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Taking in part of north-eastern and central Louisiana, the seat was the subject of some controversy earlier this year when a redesign of the district by the Republicans was ruled unconstitutional by the US Supreme Court.
Moore and Firment are vying to replace Republican Julia Letlow, who left the seat to successfully run for Senate.
A black woman, Moore has served in the state's House of Representatives since 2019.
Firment — a white man — was elected the same year. He gained prominence in 2023 for sponsoring a highly controversial measure that successfully banned gender-affirming healthcare for transgender children.
The measure bans hormone treatments and puberty-blocking drugs, gender-affirming surgeries and other related care for anyone under the age of 18.
Twenty other US states have passed similar measures but Louisiana's move drew considerable attention at the time after the Democratic governor's veto of the measure was overturned by the state's House.
At the time, Firment said Louisiana was simply following the lead of "every single southern state" that had passed similar legislation.
"We cannot allow Louisiana to become a sanctuary state for the sterilisation of innocent children," he said in a statement before the override vote.
Firment has also sponsored legislation that allows protesters to be forcibly removed from churches and other places of worship.
A total of 51 US house races are yet to be called, according to the Associated Press.
— With additional reporting by the Associated Press and Reuters news agencies.
