> America desperately needs a national internal affairs department ith an entirely separate reporting line whose boss answers directly to the president (or at least state governors).

The closest thing (and its fairly close, though it doesn't answer to the states or the President directly) is the US Department of Justice, particularly the Special Litigation Section of the Civil Rights Division, which addresses "1) the rights of people in state or local institutions, including: jails, prisons, juvenile detention facilities, and health care facilities for persons with disabilities; 2) the rights of individuals with disabilities to receive services in their communities, rather than in institutions; 3) the rights of people who interact with state or local police or sheriffs' departments; 4) the rights of youth involved in the juvenile justice system; 5) the rights of people to have safe access to reproductive health care clinics; and 6) the rights of people to practice their religion while confined to state and local institutions." [0]

But weakening oversight of local law enforcement by DoJ was a major early priority of the current Administration, so it hasn't been particularly active in the particular role you are concerned with for the last few years.

> If Biden is looking to score some points this would be a good way.

I can't really imagine a Biden Administration not ramping activity regarding law enforcement oversight back up.

[0] https://www.justice.gov/crt/special-litigation-section