Donald Trump's campaign has blamed a computer problem after a prominent white nationalist was included on a list of the Republican presidential candidate's potential California delegates.
Officials said the name of William Johnson had been withdrawn and a corrected list resubmitted to state officials.
Mr Trump's California director, Tim Clark, said a "database error" was at fault.
The campaign said Mr Johnson was removed from the list in February.
Mr Johnson, leader of the American Freedom Party, said he had received an email from Mr Clark informing him that his name had been "erroneously listed" as a delegate.
In California, Republican candidates pick potential delegates to the party's summer convention.
They are selected based on the outcome of voting in the state's June 7 primary.
The revelation is an embarrassment for a candidate who has been criticised before for being too slow to distance himself from former Ku Klux Klan leader David Duke.
Mr Johnson runs the American National Super PAC, which made automated phone calls supporting Mr Trump's candidacy across the country.
The Los Angeles lawyer said it had been a mistake for him to submit his name for consideration.
"I was naive. I thought people wouldn't notice, and if they did notice I didn't think it would be a big deal."
Mr Johnson said he never disclosed his white nationalist beliefs in his application.
"You answer the questions that they ask, and they asked: 'What have you done to support Donald Trump?'. They didn't ask: 'Are you a white nationalist?'"
Mr Trump "wants to build the wall (along the border with Mexico). He wants to cut off illegal immigration, and he wants to cut back on foreign trade, bring jobs back to America", Mr Johnson added.
"We believe Donald Trump will help lead the country in a proper direction."