Georgia Democrat Stacey Abrams admitted defeat in her effort to become the first black female US governor on Friday, but said she planned to sue the state over voting problems.
Ms Abrams' announcement effectively hands victory to Republican Brian Kemp in a bitter race that has drawn national attention both for her historic campaign and for Mr Kemp's role as the state's top election administrator.
Ms Abrams accused Mr Kemp of using his position as Georgia Secretary of State to interfere with the vote - an allegation he has strongly denied.
"To watch an elected official who claims to represent the people in this state baldly pin his hopes for election on the suppression of the people's democratic right to vote has been truly appalling," Ms Abrams told supporters in Atlanta.
"So let's be clear: This is not a speech of concession, because concession means to acknowledge an action as right, true or proper."
State officials were expected to certify Mr Kemp as the winner as soon as Friday night.
"The election is over and hardworking Georgians are ready to move forward," Mr Kemp said on Twitter.
"We can no longer dwell on the divisive politics of the past but must focus on Georgia's bright and promising future."
Mr Kemp, 55, resigned his post as Secretary of State after the election and did not oversee the final ballot review.
Ms Abrams, 44, had considered a court challenge to force a runoff, which is allowed by state law if neither candidate gets a majority of the vote. Initial results showed Mr Kemp just over that threshold.
Instead, she said she would sue the state for what she called "gross mismanagement of this election" and to protect future elections. She said she would start a new organization, Fair Fight Georgia, to advocate for voting rights.
Voters in Georgia reported long lines at polling places as aging technology was swamped by the volume of people seeking to cast ballots. Prior to election day, groups had sued to prevent Mr Kemp from throwing out more than 50,000 voter-registration forms that had been put on hold because personal information did not exactly match state databases. The lawsuit alleged that the "exact match" law disproportionately affected black voters.
Recount in Florida
Ms Abrams's announcement comes as several other races across the country remain unresolved.
In neighbouring Florida, election officials were slogging through a hand recount of ballots to determine who won a hard-fought US Senate contest.
Outgoing Republican Governor Rick Scott held a narrow lead over incumbent Democrat Bill Nelson after an electronic recount was completed on Thursday. Officials have until noon on Sunday to tally any votes missed by electronic voting machines.
Mr Scott's campaign has called on Nelson to drop out, saying it was mathematically impossible to make up the difference of about 12,600 votes, 0.15 percent of the more than 8 million ballots cast. Nelson's lawyer, Marc Elias, has said he expects Scott's lead to ultimately disappear as the recount continues.
On Friday, a federal judge rejected one of the limited paths to carry Mr Nelson's chances forward. US District Judge Mark Walker in Tallahassee denied an appeal by Mr Nelson's supporters to allow mail-in ballots that arrived after the state deadline to be counted.