Former Obama White House counsel Bob Bauer has lifted the lid on how he helped then-Democratic presidential candidate Joe Biden prepare to take on then-President Donald Trump in the 2020 election debates.

Bauer got into the role to become as personally insulting and unhinged as Trump can be in mock debate sessions, he wrote in his new book, The Unraveling: Reflections on Politics Without Ethics and Democracy in Crisis .

The lawyer watched hours of tapes of the 45th president, as a businessman, a 2016 candidate, and then in office, and read transcripts of his extemporaneous remarks on every conceivable topic, he recalled in passages shared by The Daily Beast .

Bauer, who is now an adviser to Biden, discovered how to get under the now-presidents skin, per Bidens sister Valerie Biden Owens. He would be at my Trump-worst as personally insulting and unhinged as Trump can be, he added.

It fell short of a full-scale impression of Trump and focused more on his verbal attacks and bluster, he said.

Trump, meanwhile, was prepped for the 2020 debates by former New Jersey Gov. Chris Christie (R), who has gone from being a longtime friend and staunch supporter of Trump to one of his fiercest critics.

Trump and Christie ended up hospitalized with COVID-19.

Trump tested positive just three days before the first debate. A second test reportedly returned negative but he announced a positive test a week later and was then in the hospital for three days.

Christie was in close contact with Trump throughout that period and has said it was undeniable he contracted COVID from the then-president. Christie ended up in the ICU.

CNN will host Bidens first 2024 debate with Trump in Atlanta on June 27. Anchors Jake Tapper and Dana Bash will moderate. A coin flip will decide which lectern each candidate stands at, and their microphones will be muted if its not their turn to talk. No audience will be in attendance. ABC News will host the second debate Sept. 10.