Washington (CNN) Republican Sen. James Lankford of Oklahoma said Tuesday that senators participating in President Donald Trump's impeachment trial have "got to be able to see" the book manuscript of former national security adviser John Bolton before they vote on whether to include witnesses in the proceedings.

"We have to resolve in a couple of days if we need witnesses or not -- this is an issue that's outstanding that we don't have a good way to be able to answer," Lankford told CNN's Dana Bash. "We can't say six weeks from now we'll be able to see it. We've got to be able to see it in advance to be able to make that decision on witnesses in a couple days."

Asked if seeing the manuscript would "be in lieu" of having Bolton testify in the trial, Lankford said that was not what he was suggesting.

"No, this is read the manuscript to be able to see if we need to call John Bolton, so that question's unanswered," he said. "We won't know how to answer that question until we get through today's testimony, a couple of days still of questions. I'm just saying this needs to be a part of our information so we can make that decision about witnesses."

On Sunday, The New York Times, citing multiple people's descriptions of an unpublished draft manuscript by Bolton, reported Trump in August told his then-national security adviser that he wanted to continue holding military aid to Ukraine until the country helped with investigations into Democrats, including former Vice President Joe Biden. Trump's purported statement, as described by Bolton, would directly tie the US military aid freeze with the President's requests that Ukraine announce investigations into his political rivals -- undermining a key pillar of the President's impeachment defense that the two circumstances are unrelated.

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