Charleston, South Carolina - A political network founded by the Koch brothers has endorsed former South Carolina Governor Nikki Haley for president as they believe she is the best candidate to "defeat" Donald Trump.

In a memo released on Tuesday, Americans for Prosperity (AFP), the network founded by billionaires David and Charles Koch, argued that Haley is the candidate that can "turn the page on today's toxic culture" and "is in the best position to defeat Donald Trump" in the Republican primaries.

"Between her surging to second place in the polls since August and being well-positioned amongst supporters of the other candidates, she is in a strong position to gather more support," the memo states.

"In addition, our internal polling consistently shows that Nikki Haley is by far the strongest candidate Republicans could put up against Joe Biden in a general election – winning every key battleground state and up nationally by nearly 10 points."

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The endorsement comes as a huge win for Haley, as AFP, which doesn't normally get involved in presidential primaries, is owned by Charles Koch – one of the biggest spenders in American politics.

Back in February, the group released a memo that hinted they were seeking an alternative to Trump, arguing that the Republican Party has been "nominating bad candidates who are advocating for things that go against core American principles".

In a response shared on social media, Haley said she was "honored" to have the AFP's support.