Republican Gov. Rick Scott is projected to defeat incumbent Democratic Sen.
Trump has dubbed the Republican results in the House and Senate a "success".
The Florida race in particular appeared to be coming down to the wire.
Republican Ron DeSantis, a former U.S. Representative backed by President Donald Trump, will be Florida's next governor, earning 49 percent of the vote on election night. NBC News puts the current vote count at 99% reporting with DeSantis 4,052,246, Gillum 3,996,906 - a difference of 55,340 votes out of over 8 million.
Steve Schale, a longtime Democratic strategist, recounted talking to a reporter 14 years ago about Democratic U.S. Senate candidate Betty Castor's narrow lost to Republican Mel Martinez. Other candidates in both parties took the traditional path of making visits to county fairs, local party dinners and civic groups to build up support across the state.
Gillum portrayed DeSantis as racially divisive, repeatedly pointing out his "monkey this up" comment and noting that a major DeSantis contributor called former President Barack Obama the N-word on Twitter and DeSantis refused to return his money.
DeSantis' primary campaign was based nearly entirely on his and Trump's mutual admiration.
In recent days, developers Don Peebles and Jeff Greene each gave $100,000 to Forward Florida, a political action committee supporting Gillum.
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Sixteen percent of Florida voters said this was the first time they had ever voted in a midterm election.
"We can not win the state of Florida if we continue hemorrhaging those kinds of margins in red counties", Schale said. That people are going out and they're voting for something and not against'. In some states, a governor's power to sign or veto congressional maps could decide the partisan balance.
Republicans, meanwhile, were eyeing a potential pickup in CT, traditionally a Democratic state, though Oregon Governor Kate Brown appeared set to prevail in what had been seen as a competitive race.
But for now, Republicans maintain their dominance in Florida - even if by the slimmest of margins. 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.
In Oklahoma, Republican Kevin Stitt defeated Democrat Drew Edmondson, dashing Democratic hopes that outgoing Republican Governor Mary Fallin's low approval numbers had provided a rare opportunity in a deeply conservative state.
Republicans, meanwhile, were eyeing potential pickups in CT and OR, both traditionally Democratic states.
"They did not send him a message", said Eric Johnson, who ran Democrat Patrick Murphy's 2016 U.S. Senate campaign.