Efforts by the White House and Senate Republican leaders to quickly bring President Donald Trump's impeachment trial to an end this week were facing uncertainty on Tuesday, as the Senate Majority Leader indicated to GOP Senators that he does not currently have enough votes to stop witnesses like former Trump aide John Bolton from being called to testify. That message on witnesses was delivered at a closed door meeting of GOP Senators, which convened immediately after the President's lawyers had ended their opening arguments, ridiculing the idea that stories in a new book by Bolton should find their way into the impeachment trial. 'Nothing in the Bolton revelations, even if true - even if true - would rise to the level of abuse of power or an impeachable offense,' argued the President's personal attorney Jay Sekulow, who confronted the Bolton story head on in arguments. 'You cannot impeach a President on an unsourced allegation,' as Sekulow told Senators the Bolton book was 'inadmissible' as evidence. But the warnings about Bolton from the Trump legal team did not immediately dissuade all GOP Senators, as just four could join with Democrats to ask for his testimony, which would short circuit efforts to end this trial later in the week. 'I’ve said that I think that Mr. Bolton probably has some things that would be helpful for us,' said Sen. Lisa Murkowksi (R-AK). 'We’ll figure out how we might be able to learn that.' Murkowski, along with Sen. Susan Collins (R-ME), and Sen. Mitt Romney (R-UT) were three GOP Senators considered to be possible votes to hear from Bolton, the former national security adviser for President Trump, whose new book reportedly raises questions about how Mr. Trump dealt with Ukraine, and the request for that government to start investigations of former Vice President Joe Biden, and his son Hunter. On conservative talk radio Tuesday night, there were also concerns aired that other Republicans considering testimony from witnesses included Sen. Rob Portman (R-OH), Sen. Lamar Alexander (R-TN), Sen. Bill Cassidy (R-LA), Sen. Pat Toomey (R-PA), and others. One of those Republicans against witnesses - Trump ally Sen. Lindsey Graham (R-SC) - warned that if Bolton is called, then Republicans would move to bring in a number of other witnesses as well. 'If you go down the road of witnesses, it's not going to be one - it's going to be many,' Graham told reporters in the Capitol. Democrats were left waiting on the sidelines, wondering what would happen with the GOP, not fully convinced that dissension in the ranks on Tuesday would translate into a defeat for the Senate Majority Leader at the end of the week. With the opening arguments for the President's side now finished, the Senate will start up to 16 hours of questions by Senators on Wednesday, which is expected to go for two days. That would bring the Senate to a showdown over witnesses on Friday. 'It seems to me the sooner we can get to a vote on conviction or acquittal, the better,' said Sen. Kevin Cramer (R-ND), who said he is ready to vote, without any new witnesses. Coming back from a campaign rally in New Jersey on Tuesday night, the President made clear his frustration with the process. “The Impeachment Hoax is just another political CON JOB!” the President wrote on Twitter.