Gillum conceded the Florida governor race.
According to NBC News, Republican Governor Rick Scott led Democratic Senator Bill Nelson by less than half a percentage point.
In Kansas, Democrat Laura Kelly defeated Kris Kobach, a staunch Trump ally, where outgoing Republican Governor Sam Brownback suffered from low approval ratings.
In addition to Wisconsin, Democrats also won governor races in three other states - Michigan, Pennsylvania and Kansas - that supported Trump in 2016, bolstering the party's hopes of capturing those states in the 2020 presidential election.
The Florida race in particular appeared to be coming down to the wire.
Former Rep. Ron DeSantis, R-Fla., left the United States Congress to run for governor and won the Republican gubernatorial nomination with President Donald Trump's support. Documents released under a subpoena in the months leading up to his election suggested that an undercover Federal Bureau of Investigation agent was staying close to Gillum.
Going into Election Day, the math favored Democrats: Nearly 23,000 more registered Democrats than Republicans had voted statewide, out of 5.2 million ballots cast early.
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In Florida, Gillum campaigned with former President Barack Obama.
Accusations of race-baiting have dogged Kemp, 55, and DeSantis, 40.
RiRi hopped on Instagram Monday and encouraged Florida locals to pull through for Gillum and music mogul Diddy's event. "Let the chips fall where they may".
As he takes office on January 8, DeSantis, a 40-year-old former congressman from Ponte Vedra Beach, will have the power to appoint three new members of the state's highest court, replacing three justices who are part of the liberal bloc and who have reached a mandatory retirement age.
This has been considered one of the most controversial elections after the passing an "exact match" policy, which requires voter details to match information on the state's database.
The Democrats have taken control of the House of Representatives, whilst Republicans have retained their grasp on the Senate. The next governor of Florida has a chance to shape the state's highest court by appointing new justices.
But the Democrats were trailing in redder areas of the state.
"They did not send him a message", said Eric Johnson, who ran Democrat Patrick Murphy's 2016 U.S. Senate campaign.
Polls before the election showed that health care and gun safety were among the most important issues to Democratic voters.
Broward election officials said repeatedly throughout the day that they didn't know how many mail-in ballots were still being counted.