Billionaire Koch brothers-backed network stops Nikki Haley spending

The decision by Americans for Prosperity Action (AFP) to pull the plug is another setback for Ms Haley as she seeks the Republican nomination.

She lost her home state of South Carolina on Saturday to former President Donald Trump, his fourth successive victory in the primaries.

But she has vowed to fight on.

In an email to staff on Sunday, the AFP president and CEO said their backing would focus on key Senate and House races in November's election rather than on Ms Haley.

"She has made it clear that she will continue to fight, and we wholeheartedly support her in this effort," wrote Emily Seidel.

"But given the challenges in the primary states ahead we don't believe any outside group can make a material difference to widen her path to victory."

Ms Haley's campaign insists it can still rely on enough incoming cash to keep going.

Olivia Perez Cubas, a spokesperson for Haley, thanked AFP for their support and said $1m had come in since her defeat in South Carolina. "We have plenty fuel to keep going. We have a country to save."

Defeat to Trump looms over Nikki Haley. So why stay in the race?

AFP had announced their endorsement and financial support in November, when Ms Haley was trying to establish herself as the clear challenger to Mr Trump.

Since then she has proven to be the most durable of all Mr Trump's opponents in the Republican race but it remains unclear how she can now find a path to victory.

It is looking increasingly likely the country is heading towards a rematch in November of Mr Trump against Democratic President Joe Biden.

-bbc