Former national security adviser John Bolton broke his silence Wednesday and slammed the Democratic Party-led impeachment of President Donald Trump as a "grossly partisan" effort.

Bolton, who is the author of an upcoming book on his experience in the White House, also said that his testimony would not have changed the outcome of the Senate impeachment trial, according to the Associated Press. He made the comments at a public appearance alongside Susan Rice, his counterpart under President Barack Obama.

While the former diplomat refused to disclose any details regarding his book, he did say he believed that the House "committed impeachment malpractice." This, the AP reported, drew "grumbling from the audience."

Democrats alienated Republican allies

The retired official also noted that the manner in which Democrats handled the impeachment "drove Republicans who might have voted for impeachment away because it was so partisan."

He also noted that, contrary to claims by Democrats who called upon him to testify, he does not believe that his testimony would have changed the final impeachment outcome — though he did not expect the Senate to vote against having him appear.

"People can argue about what I should have said and what I should have done," Bolton said. "I would bet you a dollar right here and now, my testimony would have made no difference to the ultimate outcome."

Rice disagreed, telling Bolton and the audience that nothing would have caused her "to refuse to share information with Congress or the public that I thought was of national import."

"I can't imagine withholding my testimony, with or without a subpoena," she added. "I also can't imagine, frankly, in the absence of being able to provide that 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."

'Implied threat of criminal prosecution'

Bolton said the reason why he did not "spill his guts" regarding the president's dealings with Ukraine is that he could have faced criminal prosecution if he had shared, even inadvertently, information that is deemed classified. When asked if he would have complied with a House subpoena, Bolton cited the White House review process that is determining whether the contents of his book can be disclosed.

"I'm not here to speculate on that with the pre-publication review process under way," Bolton said, which drew laughter from some in the audience. "Laugh all you want. This is the judgment of my counsel, somebody I worked with 35 years ago, 30 years ago at the Department of Justice."