In the end, Trump's candidate won.
But DeSantis, who stepped down from his Northeast Florida congressional seat in September, said "a lot of our super voters have yet to vote", referring to loyal Republicans who have voted in the last three or four elections.
Gillum took the risky step early in his campaign of unveiling a plan to add $1 billion in education funding - by raising the corporate tax rate.
DeSantis, 40, was considered an underdog before Trump tweeted his support for DeSantis in December, a month before he got into the race. The day after Gillum won the Democratic primary, DeSantis said Florida voters shouldn't "monkey this up" by electing Gillum.
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.
Democrats, playing catch-up after a net loss of 13 governorships and more than 900 state legislative seats during the eight-year Obama administration, fielded their largest slate of legislative candidates in more than three decades.
"The true story of this race is intense gratitude to President Trump".
"I want to encourage you not to give up", Gillum, the 39-year-old mayor of Tallahassee, told supporters at his election night party, where small groups of people embraced, tears streaming down their faces.
Identity politics factored into this campaign in a way few expected before the August primary.
Gillum was favored by polls to win the state, thanks to his strong support from the African American community and young people. The president also used his Twitter account to attack Gillum, calling him "a thief" who is "mayor of poorly run Tallahassee".
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Trump again played a major role in the general election, appearing at rallies with DeSantis in Lee County and Pensacola in the final week of the campaign.
"From an economic perspective and a results perspective", DeSantis said, Trump's message is a "good message for folks". "I always vote, but ever since Trump was elected, it's felt especially important to participate".
Both candidates have seen their share of controversy. Gillum has never been named as a subject of the investigation.
Watch the clip above, via Fox News.
The election, however, came down to those quintessential Florida constants: Base turnout and the disposition of the No-Party-Affiliated voters. He says DeSantis is "picking up where Rick Scott left off". "At least in this election".
Agriculture Commissioner Adam Putnam, DeSantis' primary opponent, put more than $30 million into his race, but lost to DeSantis, who spent around $16 million.
Mr Gillum would concede the race two hours later. The gubernatorial finalists in 2018 weren't supposed to be there in the first place. Gravis shows Nelson ahead by 3, NBC News/Marist by 4, Quinnipiac by 7, Emerson by 5, and St. Pete Polls by 4.
"I will protect Florida's future, particularly Florida's economic momentum".
"We could not be prouder of the way that we ran this race".