Donald Trump has the support of 47% of likely voters in California and his top rival in the race for the Republican presidential nomination, former California Governor Arnold Schwarzenegger, has 14%.
This puts Mr Trump in the lead in the state, according to a poll released on Monday.
“I think we’re in a very good position in California,” Mr Trump said on Monday, adding that he had the support “of almost half the state”.
The survey of 5,100 people was conducted by Edison Research for the New York Times, which said that Mr Trump was supported by 53% of registered voters in the Golden State and Mr Schwarzenegger by 43%.
Mr Trump’s lead in California is largely due to Mr Schwarzenegger’s popularity among the state’s younger voters.
Mr Schwarzenegger, who had lost his job as governor in 2014 and later resigned, has been polling well in California, particularly among women aged 35 to 49.
Mr Trump has consistently struggled with women voters, and this was evident in his performance in the contest in California’s first presidential primary in May.
He trailed the front runner, former US President Barack Obama, by 10 points in a Fox News poll, with 38% of women voting for him, compared with 46% of men.
Mr Obama has said he is still planning to seek the presidency in 2020.
California’s Democratic governor, Gavin Newsom, who was also seen as the front-runner in the 2016 race, is also campaigning in the US state and is running as an independent.
California is currently one of the few states where independents can vote for the president.
Mr Newsom is campaigning for his second term, and Mr Trump is campaigning to win the state for him.