Sen. Susan Collins, a key moderate Republican, said the new revelations leaked from former national security adviser John Bolton's forthcoming book bolster the need to call witnesses to testify at President Trump's impeachment trial.

"The report about John Bolton's book strengthens the case for witnesses and have prompted a number of conversations among my colleagues," Collins said Monday.

Collins has already indicated she is likely to vote in favor of calling new witnesses, as she did in the 1999 impeachment trial of President Bill Clinton.

Bolton, in his book, said Trump preferred withholding $391 million in security aid to Ukraine until the government pledged to investigate former Vice President Joe Biden and Democratic interference in the 2016 campaign.

The Senate is expected to vote on whether to call witnesses later this week.

Republicans have said if Democrats call Bolton or other witnesses, Trump must be allowed to call his own witnesses, among them Hunter Biden.