It is my understanding that the current standard allows a side to call two witnesses to rebut one. That works for me. Witnesses are to Trump's advantage here, which is why the House Judiciary Committee failed to call any. At a minimum, put Ciaramella, Schiff, and Yovanovitch under oath and grill them until they cry.
The Dems have nothing, and they know it. And they also know that Bolton won't help. And once the primaries get here, and Biden falls off the map, they won't even be able to make the case that he is a political opponent of Trump.