Ex-FBI head James Comey, who was sacked by President Donald Trump, claimed 45 is losing his power and called on Americans to challenge Trump.
"What's it like to be personally and publicly attacked by the president of the United States? Like many others in and out of government, I have some experience", the former FBI director wrote in The Washington Post, adding that the president's power is "shrinking its source".
Comey also slammed the president for his policies, claiming that Trump is jeopardising the country.
"I don't mean to suggest Trump is not dangerous. The horrific betrayal of allies in northern Syria demonstrates that an impetuous and amoral leader can do great harm, even in shrunken form", the ex-official stated.
The US president previously called Comey a disgrace and the worst FBI Director in the history of the bureau, slamming him for a poor investigation into the alleged Russian meddling in the 2016 vote and the Hillary Clinton private mail server scandal.
In response, the ex-FBI chief has repeatedly slammed Donald Trump since then but also criticised the current impeachment proceedings against the US president.
Washington accused Moscow of meddling in the 2016 presidential elections, but after a two-year investigation by Robert Mueller as special counsel for the US Department of Justice, the American authorities did not find sufficient evidence to prove the Trump campaign had colluded with Moscow.
The US president has repeatedly called the investigation a "witch hunt", launched by his opponents, while Moscow stressed the allegations were baseless and absurd.