Recently published allegations from former national security adviser Amb. John Bolton make it "impossible" for any senators who claim that they are "seeking the truth" in the Senate impeachment trial not to call him as a witness, Sen. Chris Van Hollen, D-Md., said Tuesday.

Appearing on "America's Newsroom" with hosts Sandra Smith and Ed Henry, Van Hollen said that all of the "momentum" in the case against the president is in favor of calling relevant fact witnesses "because the purpose of a trial is to get to the truth about what happened."

"But, this is a major test of whether or not people want to have an impartial trial because everyone in America knows that when you have a trial, and it's a fair trial, people get to call relevant fact witnesses and get relevant documents," he said.

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This weekend, a report in The New York Times revealed an excerpt from the forthcoming book titled "The Room Where It Happened" – an apparent reference to a song from the Broadway musical "Hamilton" – that could prove pivotal in the impeachment trial against President Trump.

In it, Bolton wrote that the president told him in August that he wanted to continue to freeze almost $400 million in security assistance to Ukraine until officials there helped with investigations into Democrats, including the Biden family.

The Department of Justice said in a statement Monday night that the New York Times report "grossly" mischaracterized the exchange.

Van Hollen told the "Newsroom" hosts that calls from some Republican senators to have the Biden family testify -- if John Bolton were to be brought into the Senate trial -- were the reason he proposed an amendment stating that Chief Justice John Roberts should make a ruling for requests for documents and relevant witnesses.

"If the goal of a trial is to get to the truth -- which is what trials are supposed to be all about -- then when you have the president's lawyers claiming that they're that, you know, John Bolton is making this up...Well, the way you deal with that is you swear him in. You put him under penalty of perjury so that if he lies to the Congress, he's subject to imprisonment," said Van Hollen. "That's why we have people testify under penalty of perjury."

FILE - In this July 31, 2019 file photo, then National security adviser John Bolton speaks to media at the White House in Washington. Bolton says he's 'prepared to testify' in Senate impeachment trial if subpoenaed (AP Photo/Carolyn Kaster)

Directed at the president's legal team, Van Hollen added that "you can't sort of obstruct justice and an investigation to protect yourself against serious allegations of wrongdoing that rise to the level of impeachment."

"So, that's why the House is doing this," he explained.

"This is not an appellate court. This is a trial," he concluded.

Republican senators have faced a pivotal moment and pressure has been mounting for at least four to buck GOP leaders and form a bipartisan majority to force the issue. Republicans have held a 53-47 majority, and a mere majority vote would be required on the question of witnesses.

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"John Bolton's relevance to our decision has become increasingly clear," Utah GOP Sen. Mitt Romney told reporters. Maine Sen. Susan Collins, a key moderate swing vote, said she has always wanted "the opportunity for witnesses" and the report about Bolton's book "strengthens the case."

Senate Majority Leader Mitch McConnell, R-Ky., appeared unmoved by news of the Bolton book, telling Republicans they would take stock after the defense team concluded its arguments.

Fox News' Gregg Re, Brooke Singman, Chad Pergram and The Associated Press contributed to this report.