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  (CNN) — An explosive New York Times report detailing an unpublished draft manuscript by former national security adviser John Bolton has added new uncertainty to this week's crucial vote to determine whether the Senate should subpoena witnesses and documents in President Donald Trump's impeachment trial, several GOP sources said.

Citing multiple people's description of the unpublished draft manuscript by Bolton, the Times reported that Bolton claims Trump told him in August that he wanted to continue holding military aid to Ukraine until the country helped with investigations into Democrats -- including former Vice President Joe Biden.

Before the report, GOP leaders were confident that they would defeat the vote this week. But now, it is less certain, according to three GOP sources.

"The witness vote was always going to be tough," said one source involved in the strategy. "The story makes that clear again."

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.

Lead House impeachment manager Rep. Adam Schiff tweeted Sunday night that "Bolton directly contradicts the heart of the President's defense."

"If the trial is to be fair, Senators must insist that Mr. Bolton be called as a witness, and provide his notes and other documents," he said.

Once the Senate is done hearing arguments from Trump's defense team, there will then be up to 16 hours of senator questions submitted in writing and then four hours of debate on whether to subpoena witnesses and documents. After the debate, the Senate will vote. If that motion gets 51 votes, then the Senate could move ahead with further votes to determine who to subpoena, including Bolton.

If 51 senators then vote to subpoena Bolton, the Senate resolution says he first must be privately deposed. After that, the Senate would have to decide whether to make him testify in public.

This means that the trial could be in limbo for sometime if the Senate decides to subpoena Bolton, especially given the legal battles that may ensue from any potential White House attempts to block the testimony. But if the Senate defeats the first motion to subpoena witnesses and documents, Trump may soon be acquitted.

Democrats quickly highlighted the Times report to bolster their calls for Bolton to testify.

In a joint statement, the seven House impeachment managers said the revelations confirm "what we already know."

"Americans know that a fair trial must include both the documents and witnesses blocked by the President — that starts with Mr. Bolton," the statement said.

Calling Trump's Ukraine conduct a "cover-up," House Speaker Nancy Pelosi tweeted that "The refusal of the Senate to call for him, other relevant witnesses, and documents is now even more indefensible."

Those sentiments were echoed by Senate Minority Leader Chuck Schumer. "It's up to four Senate Republicans to ensure that John Bolton, Mick Mulvaney, and the others with direct knowledge of President Trump's actions testify in the Senate trial, Schumer tweeted.