DEMs are somewhat in a box.
Lets say the trial was flawed, a sham as Shiff and Schumer say. Are the DEMs going to convict following a sham trial?
With that said if I were Warren, Sanders, Kobachar and Bennet I would vote against new witnesses so that I could get back on campaign trail. I would explain my vote with it's more important to be out campaigning for a change than to be sitting in the Senate chambers with a known outcome of acquittal.