Nikki Haley won the Republican presidential primary in Washington DC on Sunday, scoring her first victory as she struggles to remain relevant in her race against Donald Trump.

Haley's symbolic victory in the nation's capital comes just ahead of a decisive day in the long US nominating process -- Super Tuesday, in which 15 states and one territory vote.

Washington is a solidly Democratic town with a small number of registered Republicans. CNN, which was among the outlets reporting that Haley won on Sunday, put it at just 22,000.

Haley took 63% of the votes in a primary held in just one place -- a downtown hotel, according to Politico, which quoted Washington party officials.

The Haley campaign put out a statement saying, "It's not surprising that Republicans closest to Washington dysfunction are rejecting Donald Trump and all his chaos."

Trump's campaign in turn released what it called a "statement on Nikki Haley Being Crowned Queen of The Swamp."

"Tonight's results in Washington DC reaffirm the object of President Trump's campaign —- he will drain the swamp and put America first," the statement said.

The Super Tuesday milestone is expected to leave Trump a hair's breadth from securing the Republican Party's presidential nomination. The former US President has already won all the early state primaries.

Super Tuesday is seen as the last real chance for Haley to overturn Trump's march toward becoming the party's nominee once again against Biden.

Read more: Trump beats Haley in South Carolina Republican primary: US media