Former national security advisers John Bolton and Susan Rice engaged in a sometimes-tense debate over Russia and the impeachment trial against President Trump, in front of a crowd of 1,500 people Wednesday in Nashville.

The pair came together for the Vanderbilt Chancellors Lecture Series entitled "Defining U.S. Global Leadership" at Vanderbilt Unversity two weeks after the Senate acquitted Trump without subpoenaing Bolton to testify.

Bolton drew the ire of Trump when he surprisingly announced in January that he would testify in the Senate if called to do so. In leaked passages from his forthcoming book scheduled to be released in March, he said Trump told him military aid to Ukraine was conditioned on whether its leaders would investigate former Vice President Joe Biden and his son, Hunter, specifically their dealings in the country.

"I can't imagine withholding my testimony," Rice said.

"I was prepared to testify, there was a vote," Bolton responded. "Multiple times where senators were against it."

"Didn't you expect that?" Rice asked him.

"No, I didn't expect that," Bolton replied. "My testimony would have made no difference to the outcome if they knew why was going on in Washington."

He added, "I sleep at night because I have followed my conscience."

On the topic of Ukraine, Bolton, who was ousted from his position last year in the middle of the House impeachment inquiry, said congressional Democrats had "made a mess of it."

He added, "I think the House committed impeachment malpractice."

At one point, Bolton elicited a few boos from the audience when he said: "If any of you are willing to go to jail give me your names after and I'll put you in touch with the DOJ."

On the Iraq war, Rice called the conflict a "massive strategic blunder."

"The Iraq war has led to some follow on consequences of great significance," she said. "We still are embroiled. I'm trying to clean up the consequences."

Bolton, who served as ambassador to the United Nations under President George W. Bush, said he often was characterized as his Iraq policymaker.

"All I can say is...if only..." he responded.

Before becoming Trump's national security adviser, Bolton said Rice gave him "excellent advice."

"Wait, I'm not sure I ever made that public," Rice quipped.

"Well, now it is," Bolton replied.

"I just didn't want you to get shot," Rice said at the end of the exchange, which elicited laughter from the crowd.

Rice, a former U.S. Ambassador to the United Nations who served as national security adviser under President Obama, said Trump's choice of actions "unsettles our allies, emboldens our adversaries, and it's going to be difficult to put it all back together."

She also criticized Trump's history of disregarding his advisers, while Bolton argued the National Security Council needed to be adjusted to fit the president's decision-making process.

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"You can have a great decision-making process, but if the president of the United States disregards his advisers and tweets at 1:30 in the morning about whatever comes to mind, it's not going to help," Rice said.

"I'm not sure there were any officials in the national security administration that were ever shy about voicing their opinions in the Trump administration, I'm just not sure their opinion ever mattered," Bolton said.

The Associated Press contributed to this report.