They are not trying a "case". They are judging the merits of whatever case is put forth by the House in whatever article of impeachment is being presented to them. The Senate, as judge, cannot be part of the prosecution effort, of which developing or introducing "new" evidence or testimony would fall. This is a House responsibility alone that the constitution expressly prohibits the Senate from being a part of. They, the Senate, can ONLY consider whatever current evidence that comprises the current article of impeachment before them. Bolton's "new" testimony is not part of the current article of impeachment evidence or testimony. It is off limits to the Senate until such a time that the House presents an article of impeachment that contains it. No one is keeping the House from doing this. They are asking the Senate to do the job assigned specifically to the House by the Constitution. The Senate should refuse.
But doesn’t the Senate write the rules for an impeachment?
Why is McConnell saying he doesn’t have the majority votes to block calling new witnesses?
The deposition and testimony of ‘witness number 18’ along with the testimonies of the other seventeen witness is the entire limit of witnesses the Senate may consult, interview, or swear in for Senate questioning.