Margaret Good beat her GOP rival James Buchanan, whose father Vern Buchanan is the district’s congressional representative, in the election of the 72nd district on Tuesday.
The victory follows a determined campaign by the Democrats to gain momentum following their shock election defeat in 2016 – and serves a harsh blow to the Republicans ahead of the midterm elections later in the year.
Florida is of particular importance to the party. Mr Trump won the district by just five points and Republicans are thought to outnumber Democrats by roughly 13,000 voters.
Ms Good secured victory with a final margin of nearly eight points and over 3,000 votes, an encouraging sign for the Democrats that they are capable of securing Republican strongholds.
“I want to thank everyone who supported this campaign. It would not have been possible without the thousands of individuals, who like me, have had enough of the divisiveness that permeates Tallahassee and gave of their time, money and talent,” Ms Good said in a statement following her win. “The voters have spoken.”
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The former lawyer’s campaign received national attention after receiving high profile endorsements from former Vice President Joe Biden and former Maryland Governor Martin O’Malley.
“This is beyond a trend. The results are in. Republicans have a real problem in this state,” Tom Eldon, a Democratic pollster who surveyed the race, told Politico.
“This is the bellwether seat…this seat in Sarasota is the Republicans’ backyard. Anytime Democrats win a seat like this it’s great for Democrats. It happened in 1992. It happened in 2006. And it happened in this seat. This is a bellwether for bad Republican years.”
Ms Good’s Republican rival also received support from Washington. Former Trump campaign manager Corey Lewandowski endorsed Mr Buchanan, alongside former deputy campaign manager David Bossie.
“Let me just say this: You guys have an election on Tuesday,” Mr Lewandowski told a crowd on Sunday, the Sarasota Herald-Tribune reported. “The fake news is going to watch you. The failing New York Times wants to write that James Buchanan lost the seat and that Donald Trump cannot carry a candidate anymore. That’s what they want. You cannot allow it.”