Former national security adviser Susan Rice chastised John Bolton, one of her successors, for not testifying in President Trump’s Senate impeachment trial.

Bolton joined Rice on stage at Vanderbilt University to give a campus lecture on "defining U.S. global leadership" on Wednesday. The discussion at one point turned into an argument over how Bolton handled himself during the impeachment trial and whether he should have insisted on testifying.

"I can’t imagine withholding my testimony with or without a subpoena," Rice, who was national security adviser under President Barack Obama from 2013-2017, said. "I would feel like I was shamefully violating the oath that I took to the Constitution — to defend the country against all enemies, foreign and domestic."

"I said I would testify if subpoenaed [by the House]. My position is exactly the same as Joe Biden," Bolton, who served in the Trump administration from 2018 to September of last year, responded.

"Except Joe Biden didn’t have firsthand knowledge about what transpired with Ukraine," Rice hit back.

Bolton said he would be willing to testify in the trial if subpoenaed, but the Senate voted against calling witnesses after the House impeachment managers selected to prosecute the case against Trump and the White House’s defense team made opening arguments.

During the House investigation into Trump, lawmakers decided against subpoenaing testimony from Bolton. Democratic leadership said that subpoenaing Bolton would have dragged the investigation through a legal battle between the chamber and the White House if Trump claimed executive privilege over Bolton’s testimony.