Former National Security Adviser John Bolton on Thursday defended the impeachment inquiry witnesses who have been attacked by President Trump, implicitly hitting back at similar criticism he has faced for his role in the process.

KXAN reported that as Bolton was speaking to a private gathering for an investment firm in Austin, Texas, he defended several of the officials who testified in the House impeachment hearings, including Alexander Vindman, William Taylor, Fiona Hill, Tim Morrison and Marie Yovanovitch.

"All of them acted in the best interest of the country as they saw it and consistent to what they thought our policies were," Bolton told the audience during a Q&A session, according to KXAN. "The idea that somehow testifying to what you think is true is destructive to the system of government we have — I think, is very nearly the reverse — the exact reverse of the truth."

TRUMP SCORCHES BOLTON, SAYS 'FIRED' HAWK WOULD HAVE STARTED 'WORLD WAR SIX'

Bolton's comments came a day after Trump unleashed a Twitter broadside against him, blasting the former aide for his hawkish reputation and general unpopularity in Washington.

"For a guy who couldn’t get approved for the Ambassador to the U.N. years ago, couldn’t get approved for anything since, 'begged' me for a non Senate [sic] approved job, which I gave him despite many saying 'Don’t do it, sir,' takes the job, mistakenly says 'Libyan Model' on T.V., and ... many more mistakes of judgement [sic], gets fired because frankly, if I listened to him, we would be in World War Six by now, and goes out and IMMEDIATELY writes a nasty & untrue book," Trump tweeted. "All Classified National Security. Who would do this?"

The Senate is likely to decide whether or not to call Bolton as a witness in its impeachment trial of Trump on Friday or Saturday — though it appears Republicans have the votes to shut down that avenue. This comes after The New York Times reported earlier this week that the manuscript for Bolton's upcoming book said Trump had in fact linked military aid for Ukraine to that country opening investigations into the family of 2020 presidential contender Joe Biden — an allegation at the heart of the impeachment inquiry.

While Bolton and his publishers have denied they played any role in leaking the manuscript to the Times, they did not dispute the veracity of its report.

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Bolton said administration officials should "feel they're able to speak their minds without retribution" on Thursday, a potential reference to Trump's comments the day before. The Trump administration has also sent Bolton's lawyers a letter saying it found classified information while reviewing the manuscript of Bolton's book. Bolton's lawyer, however, denied that and asked the administration to expedite its review of the chapter about Trump's actions on Ukraine as Bolton could soon be called as a witness in the impeachment trial.

Sen. Lindsey Graham, R-S.C., warned against attacking Bolton's credibility earlier this week, saying such comments could lead moderate Republicans to decide they want to hear from him before casting a final vote on removing the president from office or acquitting him.

"However, I am concerned when John Bolton's credibility is attacked, it makes it more likely that some will feel the need to call him as a witness," Graham said in a statement Wednesday. "In that event, it would be important for the President and his team to call witnesses on other issues."