Donald Trump, Nikki Haley head-to-head for Republican primary in Washington

The Republican primary is set to take place on Sunday in Washington, DC, where presidential contenders Donald Trump and Nikki Haley will face off against each other.

Although this is a small-stakes contest, it could represent one of Haley's best chances to score a win over the former president.

Although the United States capital city only sends 19 out of 2,429 delegates to the Republican National Convention in July, where the nominee is formally selected, the results of the DC primary could be significant, Reuters reported.

According to a high-ranking official at SFA Fund, the main super PAC supporting Haley's bid, DC could be unusually fertile territory for her.

Despite Trump's apparent lead in the previous nominating contests, this race could be anyone's game

It will be interesting to see how things unfold on Sunday and what impact the results may have on the trajectory of the race.

The local party's single polling station in a downtown hotel has been open during daytime hours since Friday morning, and it will close for the last time on Sunday at 7pm (0000 GMT), after which votes will be counted.

The city is 100% urban, and a relatively high proportion of residents hold a college degree. The core of Trump's base skews rural, and he is particularly strong in areas with low educational attainment.

Haley campaigned in DC on Friday, hosting an event in the same hotel where the polling station is located.

Trump's campaign has been largely absent in the city, beyond sending out texts to local supporters asking them to vote.

The Democratic primary in Washington will be held in June.

On Tuesday, 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. Known as Super Tuesday, 874 Republican delegates will be up for grabs.