Florida will hold a machine recount of votes in its neck-and-neck races for the US Senate and governor.
The results will be due by 3pm local time on Thursday, Florida's secretary of state says.
The two contests, along with those for governor in Georgia and for the US Senate in Arizona, are the most high-profile races still undecided after Tuesday's congressional elections.
In Florida's election for the US Senate, Republican Governor Rick Scott had seen his lead narrow over incumbent Democratic US Senator Bill Nelson to about 12,500 votes, or 0.15 per cent, by Saturday afternoon.
In the contest for Florida's governor, unofficial results showed Republican Ron DeSantis' lead over Democrat Andrew Gillum had slimmed to about 33,700 votes, or 0.41 per cent.
Accusations of fraud and lawsuits have emerged over the Florida contests in recent days, conjuring memories of the state's 2000 presidential recount.
In that vote, the winner of the White House hung in the balance for weeks before the US Supreme Court stopped the counting and Republican George W Bush triumphed over Democrat Al Gore.
President Donald Trump has accused Democratic election officials in Florida's Broward and Palm Beach Counties of corruption, without providing any evidence.
"Trying to STEAL two big elections in Florida! We are watching closely!" Trump wrote on Twitter on Saturday during a visit to France.
Scott has filed lawsuits against Democratic election supervisors in the two counties, accusing them of violating election law and demanding access to their vote tallies.
Nelson also filed a motion in federal court asking that provisional and absentee ballots not be rejected because election officials deem that the signatures do not match voters' signatures on file.
The recounts and possible legal challenges mean it could be weeks until a winner is determined in either race.
In Tuesday's elections, Democrats won a majority in the US House of Representatives after eight years in the minority, while Republicans expanded their two-seat advantage in the Senate.