The argument that the House Democrats have been pushing in favor of calling witnesses is that it will resolve a factual disagreement.  The Democrats claim that Trump withheld military aid in order to pressure Ukraine to investigate Joe Biden and his son Hunter.  The Republicans claim that the evidence shows that the Ukraine officials were never pressured and in fact did not even know that the aid had been withheld until an August 28 article in Politico revealed that a hold had been placed on the aid.  The House managers claim that if John Bolton testifies to what is alleged to be in his book, it will prove that Trump blackmailed Ukraine to start the desired investigations.  But there is a clear distinction between what the House managers are alleging and what Bolton is alleged to have written.

The problem with this argument is that telling his national security adviser at some point that he is holding the aid while awaiting the start of investigations is not the same as demanding them from Ukraine.  In fact, the evidence from the hearings was overwhelming that the officials were never told about the hold and thus could not have been pressured by it.  Evidence at the trial revealed contemporaneous holds on U.S. aid to south Korea, Guatemala, Honduras, El Salvador, and Lebanon.  These delays were accompanied by specific demands and explanations of what the United States was asking for in return for the release of the funds.  No such demands were issued for the release of the funds to Ukraine.  If Trump had intended to use the aid as leverage to obtain the desired investigations, he would have had to inform the Ukraine officials that he was doing so.

When decisions are delayed, it is generally because the decision-maker is undecided.  There is evidence from various officials that Trump was concerned about burden-sharing.  That has been a theme of his foreign policy.  He has also been skeptical about spending on foreign aid, and there is evidence that he has put pressure on State Department officials to look into how generally the aid is being spent.  We also know from the July 25 phone call that Trump was concerned about Ukraine's actions during the 2016 election and also the connection of the Bidens with corruption in Ukraine.  We also know from his rhetoric about foreign policy that he wants to reduce the U.S. role in military conflicts over issues that he feels don't directly impact U.S. national interest.

At different times, he expressed these concerns both publicly and privately.  Trump has a tendency to think out loud and to bounce his opinions of the moment off his various advisers.  It is quite possible he indicated to Bolton at some point that the hold on aid was connected to the requested investigations.  But he clearly at other times told aides it was tied to burden-sharing as well as concerns about corruption in general.  We don't really know which one of his concerns was most important, but we do know that in the end, the aid was released.

There is a huge distinction between withholding the aid as a club to be used to demand an investigation of the Bidens and a temporary hold on aid while Trump worried about a number of issues before making the decision to release it.  Trump did ask President Zelensky to investigate Ukraine's involvement in the 2016 election and the possible corruption of the Bidens and their dealings with Burisma.  What's more, he has been emphatic in believing that he was justified in doing so.

The only real questions before the Senate is whether it was appropriate to ask for the investigations and, if not, was the offense serious enough to warrant Trump's removal from office?  Given what Bolton is alleged to have written in his book, it is hard to see how his testimony will add anything important to what is already known.