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."