Whistleblower protection was established by law in 1989. It shields a federal employee from premature identity in investigation of federal corruption. It was overwhelmingly supported by Republicans and signed into law by President Bush.
Whistleblower protection was upheld by the Supreme Court in 2014. The Federal Witness Protection Program was established by law in 1970 and signed by President Nixon. This law also protects a witness’s identity and security in formulation of a case against corrupt organizations.
This is not in conflict with the 6th Amendment’s provision to confront one’s accuser, until the final trial. And although an impeachment trial is not subject to the same laws for criminal trials, Chief Justice John Roberts would chair the Senate during the impeachment trial in order to ensure a proper balance between the rights of the accused and the rights and safety of witnesses.