The problem is that 95 Senators want to hear John Bolton (and you can bet they've all been told EXACTLY what he is going to say), ZERO Senators want to hear Hunter Biden (because they are terrified of what he might say).
So, if Mitt Romney wants to hear from John Bolton, he has to vote to also allow President Trump to call any witness that he wants to call including the Bidens, the whistleblower, ICIG, Schiff, Schiff's staff, etc.
And just what do you think John Bolton is going to say?