Key Donald Trump rival Nikki Haley has won the Washington DC Republican primary, her first victory in the nominating process and a symbolic win in the US presidential election race.

Haley, Trump’s only remaining challenger in the race for the Republican presidential nomination, won 62.9 per cent of the vote on Sunday (local time), compared to 33.2 per cent that went to Trump, Edison Research said.

Despite that, she still faces near-impossible odds in her quest to win the nomination to take on likely Democratic nominee President Joe Biden in November.

Trump won the first eight nominating contests by significant margins before losing to Haley in America’s capital city. The former president is also expected to win almost all the remaining nominating contests, according to polls.

“It’s not surprising that Republicans closest to Washington dysfunction are rejecting Donald Trump and all his chaos,” Haley spokesperson Olivia Perez-Cubas said.

Washington DC is 100 per cent urban and a relatively high proportion of residents hold college degrees. The core of Trump’s base skews rural, and he is particularly strong in areas with low educational attainment.

The city is also home to a significant number of federal workers who Trump allies have pledged to fire en masse and replace with loyalists if he wins in November. Some federal workers endured an increase in death threats in recent years, and Trump often refers to the DC area as the “swamp”.

His campaign repeated that claim as it reacted to Haley’s victory.

“Tonight’s results in Washington DC reaffirm the object of President Trump’s campaign – he will drain the swamp and put America first,” Trump press secretary Karoline Leavitt said.

“The swamp has claimed their queen.”

Haley, a former US ambassador to the United Nations and a former governor of South Carolina, will pick up 19 delegates from her win. That is just a small portion of the 1215 delegates needed to clinch the nomination.

Her victory could inoculate her from criticisms that she is unable to win a single nominating contest, though some Republicans will see her popularity in Washington as a negative. Many party leaders – Trump included – portray the city as crime-infested and run by out-of-touch elites.

This is not the first time Republicans in the capital have rejected Trump. During the last competitive Republican nominating contest in the District of Columbia, in 2016, Trump received less than 14 per cent of the vote and no delegates, even as he went on to win the nomination nationally.

The DC vote came a day ahead of so-called “Super Tuesday” when voters in 15 states and one US territory will caucus or go to the polls on the biggest day of nominating contests in the presidential primary. In all, 874 Republican delegates will be up for grabs.

The Democratic primary in Washington will be held in June.

– with AAP