Due process is all about fairness. It’s the only concept repeated twice in the Constitution. The Fifth Amendment and the 14th Amendment require all levels of government to operate within the law and to provide fair procedures. In addition, the Sixth Amendment guarantees the right to confront your accuser.
Now, some Democrat lawyers are arguing the Sixth Amendment only applies to criminal cases, not to impeachment. Maybe. The courts may need to rule on that.
But the basic laws of fairness – and common sense – dictate that in an action as serious as impeachment, a president should be entitled to a fair and open process. What we’ve seen so far looks more like a Kangaroo Court.