Republicans have no concrn whatsoever against this threat because the Dems will do just what they do anyway.
Even if they allowed witnesses, the Democrats would find some other aspects of the procedure to bitch about, and proclaim that the Senate trial was a sham.
Ironies abound, as so many of us considered the House impeachment process to have been a sham job.
The evolving liberal view, as espoused by Pelosi and others, is that the Senate trial has not been conducted properly, and because of that, the resulting acquittal of Trump will be improper.
Which is lie,cheat, steal ... even kill.