WASHINGTON – Senate Majority Leader Mitch McConnell told Republicans in a closed-door meeting Tuesday that they did not have the votes to block additional witnesses from being called in the president’s impeachment trial, according to multiple media reports.

The revelation, first reported by the Wall Street Journal, means that a host of witnesses could be considered as part of the trial, potentially lengthening the proceedings significantly.

Allowing witnesses would give Democrats a major win. Democrats hold 47 seats in the Senate and have been attempting to attract at least four Republicans to voting with them, which would give them the 51 votes needed to consider additional witnesses and documents.

If there are 51 votes, senators could propose hearing from a variety of witnesses and asking for a host of documents. It could open the door for both Democrats and Republicans to call those at the top of their list, including John Bolton, the president’s former national security adviser, Mick Mulvaney, Trump’s acting chief of staff, along with GOP-witnesses, such as the whistleblower whose complaint helped launch the impeachment inquiry and Hunter Biden, the former vice president’s son.

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A senior Republican aide noted the headlines and pointed out that a lot could change in the days before the vote on witnesses, which currently is planned for Friday.

A number of moderate Republicans have publicly acknowledged a willingness to hear from additional witnesses, something that Senate leadership was hoping to prevent against.

The most vocal has been Sen. Mitt Romney, R-Utah, whose comments about the matter only intensified after a manuscript of Bolton's book was leaked to the media and included details that the president personally told Bolton that $400 in military aid for Ukraine was tied to investigations that could help him politically.

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"I'd like to hear from John Bolton," Romney told reporters Tuesday, adding that he believed that witnesses should be paired, one from Republicans and one from Democrats. "I think if you're going to have one side call witnesses, the other side 'ought to be able to do the same."

Republicans have been particularly wary of opening the door to witnesses, both due to the uncertain nature of what people like Bolton could say and also due to the circus it would create in the more-reserved Senate.

If the Senate votes to consider witnesses later this week, it would open the door to votes on witnesses ranging from Bolton to the Bidens and the whistleblower. Democrats have railed against the prospect of calling witnesses such as the Bidens, saying the testimony would have nothing to do with the charges against the president.

Romney is one of the Republicans supporting a possible one-for-one deal, which would allow each side to call for one witness. So far, Democrats have said they are not in support of such a deal.