Then-US National Security Adviser John Bolton speaks during a media conference in Kiev. John Bolton arrived to Kiev to meet with the top Ukrainian officials.

WASHINGTON — While the fight over President Donald Trump's impeachment took a new turn Wednesday in the Senate, outside the chamber, the battle over whether former national security advisor John Bolton would testify — and what he might say if he did — heated up.

According to Bolton's upcoming memoir, "The Room Where It Happened," Trump told him in person last summer that he planned to withhold nearly $400 million in U.S. foreign aid to Ukraine until the country agreed to launch investigations into Trump's political rivals, most notably, former Vice President Joe Biden and his son Hunter.

The relationship between the stalled aid and the investigations Trump told Ukraine's president he wanted is at the heart of the impeachment case. The chamber is expected to vote Friday on whether or not to allow witnesses such as Bolton to testify at the president's trial.

On Wednesday morning, Trump, who has denied wrongdoing, issued a series of tweets excoriating Bolton. He also claimed without evidence that Bolton's book was "nasty & untrue."

TRUMP TWEET

The Senate on Wednesday began the process of questioning defense lawyers and the House managers prosecuting the case. Shortly before the questioning began, the White House released a letter to several news outlets that was dated Jan. 23, almost a week ago, written by a senior records staffer on the National Security Council and addressed to Bolton's lawyer.

The letter said that an NSC review of Bolton's book found that it "appears to contain significant amounts of classified information." The letter went on to state that "the manuscript may not be published or otherwise disclosed without the deletion of this classified information."

Bolton's attorney, Charles Cooper, did not immediately reply to an email from CNBC seeking Bolton's response to the White House letter or the president's comments about his book.

But Cooper said Sunday that the manuscript had been submitted to the White House's National Security Council for review despite "our firm belief that the manuscript contained no information that could reasonably be considered classified."

The question of whether the manuscript contains classified information could become a pivotal one if senators vote to call witnesses in the trial, and Bolton is subsequently subpoenaed to testify.

Also on Wednesday, House Foreign Affairs Committee Chairman Eliot Engel, D-N.Y., revealed for the first time that he had a conversation with Bolton in late September of last year. During that talk, Engel said, Bolton raised suspicions about Trump's abrupt firing in May 2019 of the U.S. ambassador to Ukraine, Marie Yovanovitch.

"He and I spoke by telephone on September 23," Engel said in a statement released Wednesday. "On that call, Ambassador Bolton suggested to me — unprompted — that the committee look into the recall of Ambassador Marie Yovanovitch. He strongly implied that something improper had occurred around her removal as our top diplomat in Kyiv."

Engel said he had not spoken publicly about this call before because it was "a private conversation," but that he told his House colleagues on the committees that led the Trump impeachment probe.

Elsewhere on Capitol Hill, Bolton — who has long enjoyed a sterling reputation within the conservative national security establishment — was the target of ire from top Republicans in the House and Senate.

Sen. Rand Paul, R-Ky., called Bolton a "disgruntled" former employee. But fellow Republican Sen. Lindsey Graham of South Carolina stopped short of attacking Bolton as Trump and Paul had done. Instead, he cautioned that if "John Bolton's credibility is attacked, it makes it more likely some will feel the need to call him as a witness."