The former White House strategist Steve Bannon has publicly claimed Donald Trump does not lie. But according to a new book, Bannon told aides: “Trump would say anything, he would lie about anything.”

The former president lies “to win whatever exchange he [is] having at that moment”, Bannon said.

Bannon is quoted in The Big Lie: Election Chaos, Political Opportunism, and the State of American Politics After 2020, by Jonathan Lemire, White House bureau chief for Politico and a host for MSNBC. The book will be published on 26 July. The Guardian obtained a copy.

Lemire’s title refers to Trump’s lie, supported by Bannon, that his 2020 election defeat by Joe Biden was the result of electoral fraud. That lie fueled the attempt to overturn the election that culminated in the deadly Capitol attack of 6 January 2021.

A far-right gadfly and provocateur, Bannon managed Trump’s winning campaign in 2016 then spent less than a year in the White House before being fired.

A source for numerous books about Trump – even saying he believed Trump had early stage dementia – he returned to the 45th president’s inner circle to play a central role in his attempt to stay in power.

This week, Mother Jones published audio recorded three days before polling day in which Bannon told associates Trump planned to “just declare victory” on election night.

Trump did not do so but Bannon continued to work to keep the president in power.

Lemire reports that Bannon promised January 6, the day when congress certifies electoral college results and therefore “an obscure date, known only by a few political junkies … would [come to] be ‘known the world over’”.

On January 6, Trump told supporters to “fight like hell” and to march on the Capitol. Authorities have linked nine deaths to the riot that followed. More than 870 people have been charged, some with seditious conspiracy.

Bannon’s role in Trump’s attempt to stay in power, including links to far-right groups including the Proud Boys and the Oath Keepers, is of central interest to the House January 6 committee.

Bannon refused to comply with a subpoena. He has since offered to testify but jury selection in his trial for contempt of Congress – a charge which can carry jail time – is scheduled for Monday.

Bannon escaped another brush with the law at the very end of Trump’s presidency, when Trump pardoned his former adviser in a case of alleged fraud.

As president, Trump was famously happy to lie. One count from the Washington Post found he did so 30,573 times in his time in power.

Regardless, in 2018, Bannon made headlines by telling ABC News Trump did not lie.

Told Trump “has not always told the truth”, Bannon said: “I don’t know that” and also said claims Trump lied were “another thing to demonise him”.

His host, Jonathan Karl, asked: “The president’s never lied?”

Bannon said: “Not to my knowledge, no.”

But Lemire writes that “even for Bannon, Trump was something new. The chief strategist told me that Trump ‘was not looking to win a news cycle, he was looking to win a news moment, a news second.’

“An at-times shell-shocked Bannon would relay to aides that ‘Trump would say anything, he would lie about anything to win that moment, to win whatever exchange he was having at that moment.’

“Entire campaign proposals had to be written on the fly, policy plans reverse engineered, teams of aides immediately mobilised to meet whatever floated through Trump’s head in that moment to defend his record, put down a reporter, or change a chyron on CNN.”