Democratic Socialists in the US netted a landmark win on Tuesday after a 29-year-old Ethiopian immigrant and political newcomer unseated a member of Congress who has held that job since before she was born.

Melat Kiros, a lawyer and PhD student, mounted a stunning upset for establishment Democrats after she was declared the winner of the Democratic primary in Colorado's 1st congressional district, which includes the state capital, Denver.

Kiros' opponent, Diana DeGette, had not yet acknowledged the results of the race on either of her two X accounts by Wednesday morning, but had received the backing of Colorado's elected Democratic Party leaders in a state that has long been a deep Democratic blue.

As of 11am local time in Denver on Wednesday, just over 81 percent of the votes had been counted, but the Associated Press, which typically calls the races in the US, declared Kiros the projected winner at midnight, indicating that DeGette's numbers have no chance of catching up.

DeGette spent three decades on Capitol Hill, and according to the watchdog Aipac Tracker, has received well over $1.6m from the pro-Israel lobbying group in that time.