Florida will hold a machine recount of votes in its neck-and-neck races for the U.S. Senate and governor, with results due by 3 p.m. (2000 GMT) on Thursday, its secretary of state said on Saturday.
The two contests, along with those for governor in Georgia and for the U.S. Senate in Arizona, are the most high-profile races still undecided after Tuesday's congressional elections.
In Florida's election for the U.S. Senate, Republican Governor Rick Scott had seen his lead narrow over incumbent Democratic U.S. Senator Bill Nelson to about 12,500 votes, or 0.15 percent, by Saturday afternoon.
Nelson said his campaign would continue taking action to ensure every vote is counted without interference or efforts to undermine the democratic process.
"We believe when every legal ballot is counted we'll win this election," Nelson said in a statement.
Scott urged every sheriff in the state to watch for any violations during the recount process as outlined in Florida law, and to take appropriate actions.
"We will not let unethical liberals steal this election!" Scott wrote on Twitter.
In the gubernatorial contest, Republican Ron DeSantis' lead over Democrat Andrew Gillum had slimmed to about 33,700 votes, or 0.41 percent.
Gillum, who is trying become Florida's first African-American governor, told reporters his team has organized a cadre of hundreds of volunteers and lawyers to move across the state and fight against voter suppression and for a fair count.
"This process is not over until every single vote is counted," Gillum said. "The outcome of this election will have consequences beyond who wins and who loses. How we handle this election in this process will have reverberations for democracy, for an entire generation of voters."
Accusations of fraud and lawsuits have emerged over the Florida contests in recent days, conjuring memories of the state's 2000 presidential vote recount. In that election, the winner of the White House hung in the balance for weeks before the U.S. 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.
Trump has noted the involvement of Nelson's lawyer, Marc Elias, a Washington-based Democrat who represented Hillary Clinton during her 2016 presidential race, describing him on Twitter as the Democrats' "best election stealing lawyer."
After the machine recount results are delivered, Florida Secretary of State Ken Detzner will order manual recounts if two candidates are separated by 0.25 percent of the vote or less.
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 U.S. House of Representatives after eight years in the minority, while Republicans expanded their two-seat advantage in the Senate.