Florida election officials have ordered a hand recount of ballots in the closely fought US Senate race between Democratic incumbent Bill Nelson and his Republican challenger, Governor Rick Scott, after a machine recount showed them divided by a razor-thin margin.
But in another tight contest, Republican Ron DeSantis appeared to secure the Florida governor's seat against Democrat Andrew Gillum when the electronic recount showed DeSantis with an 0.41 percentage point lead, outside the threshold to trigger further recount.
Under state law, the Florida Department of State must trigger a manual recount if an electronic recount of ballots finds a margin of victory less than 0.25 per cent.
Gillum, who initially conceded on election night but then reversed course, signaled that he had not yet given up.
"A vote denied is justice denied -- the State of Florida must count every legally cast vote," Gillum said in a statement after the machine recount concluded.
In the Senate race, Nelson trailed Scott by about 12,600 votes, or 0.15 per cent of the more than 8 million ballots cast following an electronic recount of ballots in the November 6 election, the state said.
The electronic recount suffered glitches as liberal-leaning Palm Beach County failed to complete the process by Thursday's deadline due to machine problems. Nelson's team said it had filed a lawsuit seeking a hand recount of all ballots in the county.
Overall control of the US Senate is not at stake in the Florida race. President Donald Trump's fellow Republicans held their majority in the chamber while Democrats took a majority in the House of Representatives.
But both the Senate and governor's races were closely scrutinised as Florida is traditionally a key swing state in presidential elections.
The Democrats' majority in the new House expanded by another seat on Thursday when the Maine Secretary of State's office declared Jared Golden the winner of a race against incumbent Republican Representative Bruce Poliquin. That race represented an early test of a new state ranked-choice voting system, designed to prevent candidates in races with three or more contenders from winning office without majority support.
Separately, a federal judge in Georgia on Wednesday ordered state election officials to count some previously rejected ballots in that state's governor's race, where ballots are still being tallied but Republican former Secretary of State Brian Kemp has declared victory over Democrat Stacey Abrams.
This year's campaigns went down as the most expensive midterm elections in US history, with some $US5.25 billion ($A7.23 billion) spent on advertising, up 78 per cent from the last midterm elections in 2014.
Australian Associated Press