The burden of proof to remove is on the prosecution. If you are a Senator that voted for more witnesses, you essentially admitted that there is not yet enough evidence for removal, i.e., you need to see more evidence before you vote.
How then, do you turn around and vote to convict anyway without seeing the additional evidence? The whole thing is ridiculous.
Someone in the media needs to straight up ask Romney this question: "You voted for more witnesses, meaning you felt that there was insufficient evidence to remove (why else the need for more witnesses?) Why then, are you voting to remove WITHOUT THAT EVIDENCE?"
Sadly, I think the truthful answer is that it's a big non-binding clown show attempting to change the subject from the, you know, actual state of the union...which is better than it has ever been.