Former national security adviser John Bolton faced a roasting from one of his predecessors on Wednesday night, in his second public appearance since the conclusion of Donald Trump’s impeachment trial.

Bolton, speaking alongside Susan Rice at a lecture series at Vanderbilt University, was taken to task by his co-speaker, who served as national security adviser under Barack Obama.

Rice criticized Bolton for refusing to testify before the House of Representatives during the impeachment hearings. It later emerged that Bolton has documented, in a forthcoming book, evidence regarding Trump and Ukraine that Democrats had sought him to provide on the record.

“I can’t imagine withholding my testimony with or without a subpoena,” Rice said at the Vanderbilt event, according to CNN reporter Jennifer Hansler.

“I would feel like I was shamefully violating the oath that I took to defend the constitution against all enemies foreign and domestic.”

Bolton refused to give testimony during the House impeachment hearings late last year. In January he said he was prepared to testify, if subpoenaed, in the Senate trial, where Republicans – as expected – voted against hearing from new witnesses.

At the Vanderbilt event, Bolton said, according to the Vanderbilt Hustler student newspaper: “I said I would testify if subpoenaed. My position is exactly the same as Joe Biden.”

Rice reportedly responded: “Except Joe Biden didn’t have firsthand knowledge about what transpired with Ukraine.”

Vanderbilt said ahead of the event it would not be livestreamed, and no audio of the event was provided online.

“When I was the sitting national security adviser in 2016, I was called to testify in front of the House select committee about Benghazi,” Rice said, according to the Hustler. “The last thing I wanted to do was take time away from my duty as national security adviser. But I did so voluntarily. I wasn’t subpoenaed.”

Bolton’s appearance came after an event at Duke University on Monday, where he said he was facing “censorship” from the White House, which he claimed was trying to block publication of his book.

According to the Hustler, Rice said of Bolton: “I don’t understanding using [White House] pre-clearance as a reason to not be forthcoming.”

Bolton, who left the White House in September, gave a series of teasers on Monday about what his forthcoming book might contain, in one instance referring an audience member to a specific chapter of the book.

“For all the focus on Ukraine and the impeachment trial and all that, to me, there are portions of the manuscript that deal with Ukraine, I view that like the sprinkles on the ice cream sundae in terms of what’s in the book,” Bolton said, according to the New York Times.

Bolton is said to have written that Trump spoke to him about withholding aid to Ukraine until they agreed to investigate Joe Biden and his son, Hunter Biden.

After being criticized for remaining coy over national security matters until publishing them in a commercially published book, Bolton seemed keen to tamp down his sales pitch on Wednesday.

“I didn’t come here to promote the book,” Bolton told the Hustler in an interview published before his onstage appearance.

“I’m not going to talk about the book in part because it’s still in the pre-publication review process. And to avoid criticism that I’m simply out talking [about] the book, which I am not.”

The New York Times has previously shared details from a draft of Bolton’s book. In it, Bolton affirmed that Trump was intent on withholding military aid to Ukraine until it helped investigate Democrats and the Bidens, supporting Democrats’ impeachment case against Trump.

Bolton is also said to have written that Trump was granting favors to the leaders of Turkey and China, claims the justice department has denied.

In January the White House, having received a copy of Bolton’s book, said it “appears to contain significant amounts of classified information”, and said it could not be published until certain details are removed. Bolton’s lawyers dispute that any information in the book could be considered classified.