The political network created by the billionaire industrialist Koch brothers announced on Sunday that it was suspending its support for Nikki Haley in the presidential primary after her latest defeat in South Carolina.

The group, Americans for Prosperity Action, had spent tens of millions of dollars trying to elevate Ms. Haley and prevent the renomination of Donald J. Trump, but it had already slowed its spending in the G.O.P. race dramatically after Ms. Haley fell short in the New Hampshire primary last month. The organization made its decision official on Sunday.

“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,” Emily Seidel, the chief executive of Americans for Prosperity Action, wrote in an email to the staff. The email was first reported by Politico.

Ms. Seidel wrote that the group would now focus on House and Senate races, adding that the conservative organization remained concerned about the political aftershocks of Mr. Trump winning the G.O.P. nomination.

“If Donald Trump is at the top of the Republican ticket, the risk of one-party rule by a Democratic Party captured by the progressive left is severe,” she wrote.

Ms. Seidel described how the last three elections had shown “what we can expect from voters who consistently rejected Donald Trump and his impact on the Republican party brand.”

Ms. Haley’s campaign had announced that it had raised $1 million in the less than 24 hours since polls closed in South Carolina on Saturday and she lost her home state. She has vowed to stay in the race through Super Tuesday, on March 5.

In a statement, the Haley campaign praised Americans for Prosperity Action as an ally.

“We thank them for their tremendous help in this race,” the statement read. “Our fight continues, and with more than $1 million coming in from grass-roots conservatives in just the last 24 hours, we have plenty of fuel to keep going. We have a country to save.”

The endorsement from the group, which was announced in November, was crucial for Ms. Haley. It came as she was trying to gain traction against Mr. Trump, particularly given how small her team was at the time. It gave her access to a direct-mail operation, field workers to knock on doors and people to make phone calls to prospective voters in Iowa and other states.

Still, despite those efforts, Ms. Haley came in a distant second to Mr. Trump in Iowa. One person close to the network said that it had focused on grass-roots voter outreach in the final stretch leading up to South Carolina, as opposed to advertising.

After word of the group’s pullback became public, Mr. Trump posted on his social media website that Charles Koch “and his group got played for suckers right from the beginning!”