That's what a character witness is for. Powell can only testify to the behavior he himself witnessed and experienced. As he is not a contractor who worked on Stevens's house, but was a public servant who served with Stevens in D.C., he is only permitted to testify about behavior he himself witnessed as part of Stevens's public service career in D.C. "Witness" has a very specific meaning in court.

If a man on trial for defrauding an elderly person has an elderly neighbor who testifies that he always mowed her lawn and cleared her walk for free, she is testifying to what she knows about the man in her own relationship with him. It doesn't mean she knows about or excuses his behavior in the alleged fraud. It means she can present, from personal knowledge, another view of the defendant which might have a mitigating effect on the verdict or the sentence.

It's the same with Powell, only he isn't an old lady who needs someone to mow his lawn. A character witness testifies to what character the defendant has displayed to him.

Testimony at a criminal trial is not equivalent to a political endorsement. A political endorsement is a letter of recommendation for a job. That's all it is. Its value, like the value of any letter of recommendation, depends on the status of the endorser.

Your attempt to conflate the two indicates you understand neither.