Can the evidence be made totally public by means outside the Senate trial? The very fact that McConnell is desperate to hide this is because most of the Trump Ukraine fudge-up has already been published in excruciating detail in the media globally, and now he doesn't want the rest of it to come out.
By making the facts public, perhaps by "leaking" them, they would become "common knowledge", a premise. So the GOP Senators can hardly pretend they don't know about it. It would be like trying to deny that the sun rises in the East or that water is wet, because it hasn't been formally presented as trial evidence. They will look EXTREMELY foolish in public trying to do so. Alternatively, accepting that the evidence DOES exist and they know it means McConnell's cover-up has failed.
None of this "looking foolish" stuff matters. The GOP Senate knows all this and they don't care, it's a simple political calculation. Trump may be a corrupt, malevolent weasel, but he is still more popular than the GOP Senate overall, and vastly more popular amongst Republicans.
Right now, if a GOP Senator wants to stay in office, their only possible move is to prostrate themselves before Trump, climb on to his rickety bandwagon, then put their hands together and pray to God that Trump doesn't drive the whole lot of them off a cliff before November. There really is not other politically viable path for them.
So they will swear up and down that water is dry, the Sun is cold, and Trump is the most innocent, masculine, sanest man in the world, even though they know it is a lie.