In his upcoming book, former national security adviser John Bolton writes that President Donald Trump told him he did not wish to release military aid to Ukraine until that country helped with investigations that could be damaging to Democrats, including former Vice President Joe Biden, according to media reports.

Since the allegations surfaced in September that Trump withheld $391 million in military aid to pressure Ukraine into conducting the probes, the president has insisted the aid was not held up for political purposes. On Monday he sharply denied Bolton's claims reported by The New York Times, Washington Post and the Associated Press.

The Times report has ramped up Democrats' call for additional witnesses in the Senate impeachment trial. The House impeachment managers and Senate Minority Leader Chuck Schumer said Bolton's reported claims directly contradict key elements of Trump's defense and that a fair trial requires his testimony.

Here is what we know about the book and the reactions to its reported content:

What Bolton says

Citing multiple people who had seen the unpublished manuscript, the New York Times reported that Bolton said Trump told him during a meeting in August that he did not want to release the military aid until Ukraine provided information related to the 2016 election, Hillary Clinton and Biden.

The Washington Post reported that two people familiar with the manuscript confirmed the Times' characterization of Bolton's account.

Bolton wrote that Trump remained bitter that some Ukrainian officials openly backed Clinton in 2016, which he conflated with a debunked conspiracy theory that Ukraine, not Russia, was behind the theft of emails from the Democratic National Committee and the Clinton campaign.

The president's personal attorney Rudy Giuliani helped stoke his concerns about Ukraine and his animosity toward then-U.S. ambassador to Ukraine Marie Yovanovitch, Bolton said.

Bolton wrote that he, Secretary of State Mike Pompeo and Defense Secretary Mark Esper unsuccessfully tried to persuade Trump to release the aid on several occasions, according to the Times.

The book, "The Room Where It Happened: A White House Memoir," is set for publication on March 17, according to Amazon.

Corruption fight:Trump's conspiracy theories thrive in Ukraine, where a young democracy battles corruption and distrust

Trump denies the claims

"I NEVER told John Bolton that the aid to Ukraine was tied to investigations into Democrats, including the Bidens," Trump said in a tweet early Monday. "In fact, he never complained about this at the time of his very public termination. If John Bolton said this, it was only to sell a book."

He repeated his claim that a rough transcript of his July 25 call with Ukrainian President Volodymyr Zelensky shows he did not act improperly – though Democrats say the call proves the exact opposite – and retweeted several posts that attacked Bolton's credibility.

White House press secretary Stephanie Grisham told Fox News on Monday that "the timing of all this is very, very suspect."

"The president did nothing wrong, and we stand by exactly what we've been saying all along," Grisham said.

Giuliani also spoke out against Bolton on Twitter.

"There is no way in the world President Trump would say this to John Bolton. It’s a shame that a man will sacrifice his integrity to make a few bucks on a book. No wonder he accomplished so little as National Security Advisor," he wrote.

Democrats insist Bolton be subpoenaed

The manuscript reports ramped up calls from Democrats for the Senate to issue Bolton a subpoena in the impeachment trial.

"There can be no doubt now that Mr. Bolton directly contradicts the heart of the president’s defense and therefore must be called as a witness at the impeachment trial of President Trump," the seven Democratic House impeachment managers said in a joint statement responding to the report.

"John Bolton has the evidence," Schumer said Sunday in a tweet calling on Republican senators to join Democrats in a vote to allow additional testimony.

Democrats have wanted Bolton to testify since the impeachment inquiry began, but the House committees did not issue him a subpoena after Bolton said he would fight it in court. Democrats claimed the legal battle would delay the impeachment too long. But after the House impeached Trump on articles of abuse of power and obstruction of Congress, Bolton said he was "prepared to testify" if the Senate issued him a subpoena.

Will witnesses be called? Handful of key Senate Republicans will determine whether to call witnesses at President Trump's trial

How will the book affect the trial?

After Trump's legal team finishes presenting its case, senators will have 16 hours to submit questions, according to the impeachment rules that were approved last week. At that point, senators will debate whether to call additional witnesses.

It will require a simple majority to approve the issuing of subpoenas. It is unclear whether four Republicans will vote with Democrats to approve such a measure or whether the report about what Bolton has to say might influence their decision. Though a number of Republicans have rejected the idea of issuing subpoenas, arguing that was the House's responsibility, others have signaled they may wish to hear more testimony.

Sen. Susan Collins, R-Maine, who said she would make her decision on witnesses after both sides had presented their arguments, said in a statement on Monday that reports about Bolton’s book "strengthen the case for witnesses and have prompted a number of conversations among my colleagues."

Utah Sen. Mitt Romney told MSBNC that it’s "increasingly likely that other Republicans will join those of us who think we should hear from John Bolton."

The news about Bolton's book did not sway Missouri Sen. Roy Blunt, who said Monday it was "the House's job" to put the case together and warned that subpoenaing Bolton "could stretch this out for weeks and months" if the administration asserted executive privilege.

Sen. Lindsey Graham, R-S.C., a staunch Trump supporter who a day earlier said he had already seen "more than enough to make my decision," conceded it could be "important" to know what was in Bolton's manuscript, according to NBC News.

Want to talk more about politics?:Join our Facebook group: ‘Across the Aisle, Across the Nation.’

Who saw the manuscript?

Bolton's attorney Charles Cooper said in a statement that they gave the White House a copy of the manuscript for review on Dec. 30, which is common practice when a current or former administration official writes about their tenure.

Cooper blamed the White House for the leak to the Times and said the article shows "the review process corrupted and that information has been disclosed by persons other than those properly involved in reviewing the manuscript."

But National Security Council spokesman John Ullyot said only the NSC had seen the manuscript.

"No White House personnel outside NSC have reviewed the manuscript," he said.

Grisham told Fox News the manuscript had been "kept walled off for obvious reasons." She said it was suspicious that "this manuscript magically appeared" after Trump's team opened its defense indicates and that the Amazon link then suddenly went up, implying it was an orchestrated effort by the book's publisher.

Bolton and his publishers, Simon & Schuster and Javelin Literary, "categorically" denied any coordination with the Times "or anyone else regarding the appearance of information about his book," in a statement.

"Any assertion to the contrary is unfounded speculation," the statement said.

Contributing: Maureen Groppe, David Jackson and Christal Hayes, USA TODAY; The Associated Press