Prominent Trump supporter and newly announced Senate candidate Joe Arpaio has doubled down on his claim that Barack Obama’s birth certificate is fake.

“It’s phoney document.” Mr Arpaio told CNN’s Chris Cuomo. "...We have the evidence. Nobody will talk about it, nobody will look at it, and anytime you want to come down – or anybody – we'll be glad to show you the evidence.”

Mr Arpaio is a former Arizona sheriff who rose to national fame for his controversial policing tactics and hardline stance on immigration – and later, for his prominent role as a Trump campaign surrogate. Last year, after the sheriff was convicted of criminal contempt of court, he became Mr Trump’s first pardon as President.

On Wednesday, the day after he announced his campaign for the US Senate seat left open by retiring Senator Jeff Flake, Mr Arpaio returned to a favourite talking point: birtherism.

“I started this because of a fake document – a government document,” Mr Arpaio said of his and Mr Trump’s now-infamous campaign to delegitimize Mr Obama’s birth certificate. The pair both claimed for years that the former President had been born outside the country, making him ineligible for office.

'I can get my family back': immigrants driven out of Arizona return home Show all 3 1 /3 'I can get my family back': immigrants driven out of Arizona return home 'I can get my family back': immigrants driven out of Arizona return home Protesters outside Arizona's State Capitol in Phoenix celebrate their legal victory against the crackdown on illegal immigration Joshua Lott/Reuters 'I can get my family back': immigrants driven out of Arizona return home Campaigners with Patricia Rosas, right, who now says she can look forward to being reunited with her daughter, son-in-law and grandchildren after the court ruling on Senate Bill 1070 Guy Adams 'I can get my family back': immigrants driven out of Arizona return home The judge's decision is a setback however for Arizona's governor Jan Brewer Jill Torrance/Arizona Daily Star/AP

While Mr Trump dropped the theory for the sake of his presidential campaign – he told the Washington Post in 2016 that he believed Mr Obama was born in Hawaii, not Kenya – Mr Arpaio has not followed suit.

"No doubt about it, we have the evidence, I'm not going to go into all the details, yeah, it's a phoney document," Mr Arpaio said.

Indeed, Mr Arpaio seems to have maintained many of his most controversial stances as he mounts his Senate campaign. Despite narrowly avoiding jail time for refusing to stop his officers from arresting people based on their presumed immigration status, Mr Arpaio is making immigration a focal point of his campaign.

“Being a US Senator is a little different than being the sheriff, because you can do a lot of things in the US Senate, and I have many plans, believe me,” he told the Washington Examiner.