A small number of responsible Republican senators—probably only three, in fact—could form a constitutional caucus. Or a small bipartisan group of senators could form such a group. They can determine what the outlines of a full and fair trial would look like. Surely this would include some number of witnesses on both sides, not all of whom need have testified in the House. Insisting on this should be easy, as it would be in accord with the wish of the House managers and the professed wishes of the president. The rules should err in general on the side of completeness and, of course, fairness.
This is the task that a small group of senators faces, and the opportunity they have: To ensure the Senate in which they have the honor to serve lives up to the high standards of our history and our constitutional order.
The awesome responsibility of an impeachment trial of a sitting president has fallen on very few senators over the course of American history. Most who have been put in this rare position found that a fair trial enabled them to win the respect of their constituents, even those who might disagree with the decision they rendered on the verdict.