McConnell tells GOP senators he currently doesn't have votes to block witnesses at Trump trial

Senate Majority Leader Mitch McConnell told Republican senators Tuesday evening that he doesn't currently have the votes to block witnesses at President Donald Trump's Senate impeachment trial. The Senate was set to vote as early as Friday on whether to consider having witnesses and allowing new evidence. Erik S Lesser/EPA via Shutterstock Senate Majority Leader Mitch McConnell arrives for work during the second week of the impeachment trial of President Donald Trump at the U.S. Capitol in Washington, Jan. 28, 2020.Senate Majority Leader Mitch McConnell arrives for work during the second week of the impeachment trial of President Donald Trump at the U.S. Capitol in Washington, Jan. 28, 2020. Erik S Lesser/EPA via Shutterstock McConnell and the GOP senators met behind closed doors shortly after Trump's legal team ended their opening arguments, in which they tried to discredit reported new allegations from Trump's former national security adviser, John Bolton. MORE: Senate impeachment trial: Trump legal team dismisses Bolton allegations The New York Times reported that in a new book, Bolton claims that Trump told him he would keep withholding U.S. military aid to Ukraine until Ukraine agreed to help investigate former Vice President Joe Biden and his son Hunter. The president's lawyer, Jay Sekulow, argued on Tuesday, "You cannot impeach a president based on an unsourced allegation." He called the Bolton book manuscript "inadmissible." MORE: Amid impeachment trial, White House counters with events showcasing Trump's agenda The development landed like a bombshell amid Trump's trial, with Democrats insisting that Bolton now must be called as a witness and even some key moderate Republicans, including Sens. Mitt Romney of Utah and Susan Collins of Maine, expressed support for the idea, but have not yet sided firmly with the Democrats. ABC News Trump's counsel Jay Sekulow presents opening arguments during impeachment proceedings against President Donald Trump at the Capitol, Jan. 28, 2020.Trump's counsel Jay Sekulow presents opening arguments during impeachment proceedings against President Donald Trump at the Capitol, Jan. 28, 2020. ABC News Even before the trial began, Democrats have been targeting four Republican senators -- Romney, Collins and Sens. Lisa Murkowski of Alaska and Lamar Alexander of Tennessee -- to get to the 51 votes Democrats need to force the Senate to call witnesses, including Bolton. A senior White House official told ABC News that the president's defense team still believes they will be able to defeat the measure to call witnesses. "We are exactly where we were going into the weekend," the official said. "There are four senators in play, two of have spoken publicly about where they stand (Romney, Collins) and two who have not (Murkowski, Alexander)." "It's still a hard vote, but we are working hard. It's a long time until Friday," the official added. Still, the Senate's number two Republican, John Thune, who is responsible for whipping the vote on witnesses, acknowledged Tuesday to ABC News that there is genuine fear that the trial could turn into a chaotic mess. "Nobody wants a wide-open, sort-of free-for-all where this thing gets bogged down for weeks on end," Thune said. He said he thought the GOP conference was unified behind a plan that would see more witnesses called than the Trump team would want, in exchange for a witness like Bolton. But Thune said it was proving difficult to figure out how to manage what could become an unwieldy process. "My assumption is that the president's counsel is going to have a fairly long list that they'll want to call, if the Dems get to have the witnesses they want to call. So, I just think it's fraught with a lot of peril and could be a long, drawn-out process," said Thune. ABC News Chief White House Correspondent Jonathan Karl contributed to this report.