"His policymaking is so incoherent, so unfocused, so unstructured, so wrapped around his own personal political fortunes, that mistakes are being made that will have grave consequences for the national security of the United States," Bolton told George Stephanopoulos Monday.
After spending 17 months at Trump's side for historic summits and daily meetings, Bolton is speaking out in his new book "The Room Where It Happened." Bolton also sat down for a special interview with ABC News' Martha Raddatz that aired Sunday.
But the reaction was swift against Bolton, who refused to testify before House lawmakers during the impeachment hearings. He later said he would appear as a witness before the Senate during Trump's trial but Republicans blocked a vote to call witnesses.
Bolton has pushed back on criticism by blaming House Democrats on what he called a rushed, politicized handling of the probe.
"The Democrats made a conscious decision at the beginning of the Ukraine impeachment effort to push Republicans aside. I think there were a lot of Republicans in the House that might have been open to a more reasoned, non-partisan effort," Bolton said.
He declined to say Monday if he would testify now if subpoenaed by Congress, instead turning attention to November's presidential election.
"The primary way we rein presidents in is not through impeachment," Bolton argued. "It's through elections and presidential behavior can be reckless, reprehensible, dangerous."
Bolton announced during Sunday's special that he will not vote for Trump, adding on Monday that he's "not switching to the Democratic Party. I'm still a rock-ribbed conservative Republican."
Instead, he will write in a conservative politician's name in November.
That contrasts with what Bolton told voters in 2016 when he supported Trump's candidacy -- saying that a vote for a third-party candidate was a vote for Hillary Clinton.
"Having watched Donald Trump for 17 months, I cannot in good conscience vote for him, and I think there are a lot of other Republicans who feel the same way. This is not a happy election for conservatives," Bolton said.
Bolton said he's laying the groundwork for the discussions within the Republican Party about "what the post-Trump party will look like," adding that will happen whether Trump wins or loses in November.
In the days since details from Bolton's book first started to leak, Trump and his allies have attacked him. Secretary of State Mike Pompeo called him a "traitor" and accused him of lying -- charges that Bolton dismissed.
"He's responding the same way that President Trump is -- they call names, they deny -- but they're not willing to face up to what the real facts are," Bolton said.
The Trump administration has also accused Bolton of revealing classified information in his book. A federal judge seemed to agree Saturday, ruling that the Department of Justice can't block the release of his book, but warning Bolton "has exposed his country to harm and himself to civil (and potentially criminal) liability."
Bolton said Monday he and his lawyers "respectfully disagree" with the judge.