Bolton served in the Trump White House for 17 months between 2018 and 2019 and left on fractious terms. He authored a book on his time in the administration which characterized the president as ignorant of basic geopolitical facts and driven solely by desire for re-election, calling Trump "erratic" and "stunningly uninformed."
Trump responded by accusing Bolton's book of being a work of fiction written in spite by a "disgruntled boring fool" and a "wacko."
Bolton told CNBC's "Squawk Box Asia" on Wednesday that it became apparent during his time in the White House that Trump had no "philosophy" or "grand strategy."
"He is not a conservative, he is not a liberal either in American terms, he is Donald Trump and his focus is Donald Trump and Donald Trump's re-election," Bolton said, adding to comments he made to CNBC last month.
"I just don't think that he ever fully understood the gravity of his responsibility as president, the weight of the decisions he had, and it is why I worry about the harm that he did to the US in his first term and why that harm could be even more profound in a second term."