From Bigun's post above:
"And no Person shall be convicted without the Concurrence of two-thirds of the Members present."
Better watch out on this one.
If some of the Republican senators decide to boycott the trial, that seems to reduce the absolute number of votes need for a conviction downward.
Example:
With 100 Senators present to vote, 67 votes are required to convict.
BUT...
If say, only 70 Senators are present, and the Constitutional language is strictly applied, only 47 votes might be needed !