The Persident refused to let them testify in the House, hence the Obstruction charge. But, the Chief Justice presides over the Senate trial and could order them to appear. If they have evidence that is relevant, why is it so important to make up excuses for having the American people and our Senators kept from hearing their testimony? Also, in a trial, prosecutors often bring in extra witnesses, in addition to those who testified in the Grand Jury process. I grew up hearing that the Republicans were the "Law and Order Party." It seems to me that by not wanting, in fact condoning the blocking of, crucial physical evidence and testimony, they are now the party of Lawlessness and Disorder.