Former college administrator Debbie Mucarsel-Powell unseated Republican Rep. Carlos Curbelo on Tuesday in Florida's 26th Congressional District.
Mucarsel-Powell beat Curbelo by 1 percentage point to take the district, which covers Monroe County and parts of southern Miami-Dade County.
Curbelo, who has served in Congress since 2012, had been favored to win despite the district's voters favoring Hillary Clinton by more than 15 points in 2016.
Curbelo has been an outspoken advocate for undocumented people who came to the United States as children, also known as "Dreamers."
In a barrage of campaign ads, Mucarsel-Powell and Democratic allies tied Curbelo to the policies of President Donald Trump, who is relatively unpopular in the heavily Hispanic district.