John Bolton, the former National Security Adviser, and other witnesses, including Mick Mulvaney at the least, need to testify in the Donald Trump Impeachment Trial, and Chief Justice John Roberts needs to step in and rule on this matter, with the help of at least three Republican Senators.
It seems as if Mitt Romney, Susan Collins, and Lisa Murkowski are ready to vote for witnesses and testimony, and also, Lamar Alexander, Pat Toomey, Rob Portman and Jerry Moran seem leaning that way.
If all of these seven support each other, or at least four, then it will occur, but if not, Roberts needs to intervene if there is a 50-50 tie in the Senate.
It may not change the final vote on conviction, requiring two thirds to remove Donald Trump, but the trial, in order not to be a sham, MUST have witnesses and testimony, and Roberts has the responsibility to intervene if necessary!