Former National Security Advisor John Bolton defended his decision not to testify in the impeachment inquiry into President Donald Trump.

Speaking at an event at Vanderbilt University on Wednesday, Bolton slammed House Democrats over their handling of the impeachment probe and argued that it was “so partisan” that Republicans that might have voted for impeachment were driven to vote against it.

“I think the House committed impeachment malpractice,” Bolton said, adding, “The process drove Republicans who might have voted for impeachment away.”

When pressed by Susan Rice, who served as former President Barack Obama’s national security advisor, about his decision not to testify, Bolton said it would not have changed the outcome.

“My testimony would have made no difference to the ultimate outcome,” he said.

“I can’t imagine withholding my testimony, with or without a subpoena,” Rice responded. She added:

“I also can’t imagine, frankly, in the absence of being able to provide the information directly to Congress, not having exercised my First Amendment right to speak publicly at a time when my testimony or my experience would be relevant. And, frankly, when my subordinates … were doing their duty and responding in a fashion consistent with their legal obligations to provide information.”

Rice was referring to other members of the National Security Council who testified about their knowledge of Trump’s decision to place a hold on roughly $400 million in military aid to Ukraine.

House Democrats considered subpoenaing Bolton to have him testify in their impeachment probe. But after he vowed to take the subpoena to court, they decided against seeking his testimony.

Speaking about Bolton’s decision to remain silent through the impeachment process, Rice said she would feel as though she were “shamefully violating the oath that I took to support and defend the Constitution.”

However, Bolton noted that his forthcoming book, about his time in the White House, is still being review by the White House to ensure that it does not include classified information. He added that he was concerned the White House would try to prosecute him if he discussed its contents before the review process was finished.

Congressional Democrats increased pressure to call Bolton as a witness in the Senate’s impeachment trial after The New York Times reported that Bolton said in a draft of his book that Trump linked the military aid to investigations of his rivals.

However, the Republican-controlled Senate rejected those calls, with some senators arguing that Bolton’s testimony would not include any new information that would sway their decision.

Trump was impeached in the House on Dec. 18 on two articles of impeachment — abuse of power and obstruction of Congress — but acquitted in the Senate. He has denied that he did anything wrong regarding the hold on the aid to Ukraine.

However, some Republican senators said they feel Trump’s actions were “improper” but not impeachable.

While one Republican senator voted to convict Trump on one article of impeachment in the Senate, no Republicans voted in favor of the articles in the House. And every other Republican senator voted to acquit Trump on both articles of impeachment.