President Donald Trump slammed his former national security adviser John Bolton on Wednesday, bashing his former top aide after he reportedly contradicted a key element of the president's impeachment defense in an upcoming book.

Trump suggested that if Bolton, a conservative hawk, were still in the White House, the U.S. "would be in World War Six by now."

Those comments came hours after another tweet in which Trump asked: "Why didn't John Bolton complain about this 'nonsense' a long time ago, when he was very publicly terminated."

Bolton asserts he was not fired.

Trump continued: "He said, not that it matters, NOTHING!"

Earlier this week, The New York Times reported that according to a manuscript of Bolton's book, Trump told Bolton in August that nearly $400 million in aid to Ukraine would not be released until the country provided all of the information it had in connection to the investigations of Democrats the president sought. The manuscript has not been seen by NBC News.

That reported conversation between the two men in which Bolton claims the president linked Ukrainian aid and the investigations contradicts the president's impeachment defense, as Trump and allies have said the hold on military aid and investigations were not linked. Meanwhile, Trump's defense team has argued that "not a single witness testified that the president himself said that there was any connection between any investigations and security assistance, a presidential meeting or anything else."

Trump said he "NEVER" had such a conversation with Bolton. In an interview with "CBS This Morning" that aired Wednesday, Trump's personal attorney Rudy Giuliani called Bolton a "back-stabber" who "should not testify" before the Senate.

On the other hand, John Kelly, the retired Marine general who served as Trump's chief of staff for 18 months, said Monday that he believes Bolton and thinks the Senate should allow for additional witness testimony.

"If John Bolton says that in the book, I believe John Bolton,” Kelly said.

In a statement Wednesday, Sen. Lindsey Graham, R-S.C., seemed to suggest that Trump should stop publicly blasting Bolton because it could increase the chance of him being called to testify before the Senate.

"The House managers’ claim that the sole reason President Trump temporarily paused the aid was purely personal and political, not public, does not withstand scrutiny," Graham said. "However, I am concerned when John Bolton’s credibility is attacked, it makes it more likely some will feel the need to call him as a witness."

"In that event, it would be important for the president and his team to call witnesses on other issues," Graham continued.

Bolton's reported claims have ratcheted up calls for his testimony to be included in the Senate trial, and a few Republicans have suggested a willingness to hear from new witnesses. At least four Republicans would need to vote with all Democrats in order for such testimony to be approved.

Trump tweeted against new witnesses Wednesday, falsely saying no Republicans witnesses testified in the House trial.

"Witnesses are up to the House, not up to the Senate," he said. "Don't let the Dems play you!"

The brouhaha comes as Trump's trial moves to the question and answer segment, soon to be followed by a vote on witnesses and documents.

Senate Majority Leader Mitch McConnell, R-Ky., acknowledged to Republican senators during a private meeting Tuesday he does not yet have enough votes to block witnesses, a GOP aide familiar with his comments told NBC News, though Republicans appeared confident they would ultimately succeed in blocking the new testimony.