Alexandria Ocasio-Cortez has won re-election to her seat in Congress for New York's 14th congressional district.
The Democrat beat first-time Republican challenger, John Cummings, who had raised more than $10m during his campaigh despite being classed as a relative long-shot.
Despite a large postal marketing campaign, the 60-year-old trailed Ocasio-Cortez by nearly 37 percentage points - and Associated Press declared her victory at 10.09pm (3.09am UK time).
Former New York Police Department officer Cummings had built his ticket as an alternative to Ocasio-Cortez, who is seen as a rising star of the Democrat's left.
Ocasio-Cortez - known colloquially as AOC - is, at 31, currently too young to run for President, but could be in-line come the next US election at 35.
Following her win, she said: "Serving NY-14 and fighting for working class families in Congress has been the greatest honor, privilege, and responsibility of my life.
"Thank you to the Bronx and Queens for re-electing me to the House despite the millions spent against us, and trusting me to represent you once more."
Though her challenger raised such a high amount during his campaign, AOC was expected to win her district - in spite of her devisive politics.
She has championed policies including universal health care, free college tuition and crackdowns on police brutality.
As of 12am (5am UK time), President Donald Trump and Democratic challenger Joe Biden are locked in tight races in battleground states after Americans voted in the US election.
Mr Biden picked up the first battleground state of the night, New Hampshire, a small prize that Mr Trump tried to steal from Democrats.
But races were too early to call in the most fiercely contested and critical states on the map, including Florida, North Carolina, Georgia and Pennsylvania.
The Democrats won California, the nation’s biggest electoral haul, and other predictable victories including Colorado and Virginia, two former battlegrounds that have become Democratic strongholds. Mr Trump’s wins included Kansas, North Dakota and other conservative bastions.
Millions of voters put aside worries about the virus – and some long queues – to turn out in person, joining 102 million fellow Americans who voted days or weeks earlier, a record number that represented 73% of the total vote in the 2016 presidential election.
