Though outgoing Governor Rick Scott claimed victory over incumbent Democrat Bill Nelson on Tuesday, a statewide vote recount has now been triggered.
The margin between the two has narrowed to within 0.5% of each other.
Florida's gubernatorial race between Democrat Andrew Gillum and Republican Ron DeSantis could also face a recount.
On Tuesday night, Mr Scott announced his victory - and was even congratulated via phone by President Donald Trump - but Mr Nelson did not make a concession speech at the time.
The next morning, Mr Nelson, who has served 18 years as senator, said in a statement: "We are proceeding to a recount."
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