Barack Obama says the rise of Trump was the racist reaction to having a “black man in the White House” in his forthcoming memoir.
The book called A Promised Land is released on 17 November and charts his political career and personal life.
In it he reflects on his rise to the presidency and how it impacted race relations in the country.
According to CNN, which obtained a copy of the book, he wrote: “It was as if my very presence in the White House had triggered a deep-seated panic, a sense that the natural order had been disrupted.
“Which is exactly what Donald Trump understood when he started peddling assertions that I had not been born in the United States and was thus an illegitimate president.
“For millions of Americans spooked by a Black man in the White House, he promised an elixir for their racial anxiety.”
The conspiracy theory Mr Obama was born outside the US, and therefore ineligible to run for the White House, first started in 2008.
More from Barack Obama
US election 2020: Barack Obama attacks Donald Trump for ‘ignoring’ coronavirus pandemic at Joe Biden rally
First volume of Barack Obama’s memoir to be released two weeks after US election
Donald Trump ‘hired Faux-Bama to mock and fire predecessor’, former lawyer claims
US election 2020: Joe Biden’s energised pitch for the White House had punch few were expecting
John Lewis funeral: Former presidents Obama, Clinton and Bush pay tribute to US civil rights hero
George Floyd death: Tragedy of last few weeks gives people the opportunity to be awakened – Obama
However, it was popularised by Donald Trump in 2011 when he famously asked to see his birth certificate.
He continued to peddle the theory on Twitter in the following years.
On Thursday, Mr Obama tweeted: “More than anyone else, I wrote my book for young people – as an invitation to once again remake the world, and to bring about, through hard work, determination, and a big dose of imagination, an America that finally aligns with all that is best in us.”
In excerpts shared with The Atlantic, Mr Obama reflects on the state of democracy in America.
He writes: “Our democracy seems to be teetering on the brink of crisis… But I also know that no single election will settle the matter.
“Our divisions run deep; our challenges are daunting.”
He added: “I have an abiding trust in Joe Biden and Kamala Harris, in their character and capacity to do what is right.”
The 768-page book follows Becoming – the successful memoir by his wife, and former first lady Michelle Obama -released in 2018. As of 2019 it had sold 10 million copies.