Democrats picked up governorships in Kansas and at least three other USA states on Tuesday but failed to win high-profile races in Florida and OH, as voters cast ballots in three dozen gubernatorial contests across the country.
Gillum conceded victory to DeSantis Tuesday night after narrowly trailing behind.
"I'm looking forward to seeing every vote counted", Gillum said on Twitter.
The campaign says it's monitoring the situation and preparing for a possible state-mandated recount.
He joked during brief remarks that he wasn't sure his wife, R. Jai Gillum, whom he met at Florida A&M University, had voted for him.
DeSantis, whose endorsement from President Trump boosted his nomination for the Republican party, pledged during his campaign to keep taxes low in Florida, and improve the coastline through beach restoration and flood mitigation.
"I gave them the number that we had, but I did not give them the names", Okaloosa Supervisor of Elections Paul Lux told the Herald/Times.
"We recognize that, you know, we didn't win it tonight".
Although the Republican candidate was declared the victor on Tuesday night, the governor's race in Florida appears close to a recount.
"For Bill Nelson - the task is getting the "win" ... no matter what", a statement from Mr. Scott's camp read.
As of 2 p.m. Thursday, DeSantis led Gillum by 38,515 votes out of almost 8.2 million cast, or a difference of 0.47 of a percentage point.
Rick Scott, who left the governor's mansion to run for the Senate, squeezed out Bill Nelson, the veteran centrist Democrat. Gillum called himself "the most unapologetically progressive" candidate in the five-candidate Democratic field and staked out some positions beyond the reach of state government, such as his call for Trump's impeachment or his endorsement of Sanders' "Medicare for all" plan.
By 2 p.m., Gillum gained on DeSantis by another 4,441 votes, and now trails by only 0.47 percent. In the months leading up to the election, Gillum was projected to win the gubernatorial bid.
Wiles said a big question mark concerned how many independent voters would turn out and if they would break for any particular candidate.
In a fierce scramble for votes, thousands of provisional ballots cast by people who did not have an ID or who voted at the wrong precinct are already the focus of scrutiny by both sides. Since the senate race crosses the threshold of the machine recount option, the political environment pretty much requires Detzner to conduct one - and as long as he's doing those two races, he may as well machine-recount the gubernatorial race, too.
It's unknown how many provisional ballots have been cast statewide in Florida this year.
Marc Elias, a lawyer hired by Nelson, said he expects the margin to continue to narrow. Osceola County Gillum volunteer Susan Wills said the secretary of state's office had not yet released information about the provisionals early Thursday afternoon.