Deputy Attorney General Rod Rosenstein should have recused himself already for his conflicts of interest in the Russia investigation as he is both a witness and friend of subjects under investigation, but he did not.
He is also guilty of knowingly using false evidence to extend a FISA warrant, under oath, to the FISA Court. He also last week in a fit of rage threatened Chairman Devin Nunes with retaliatory subpoenas of his texts and phone calls if he persists in investigating the DOJ and FBI. The House Intelligence Committee has Constitutional authority to oversee the DOJ and FBI. Neither the DOJ or FBI are Constitutionally mandated branches of government. They were created by Congress and as such, are subject to Congressional oversight of their activities and their funding.
Jeff Sessions may have recused himself from the Russia investigation, but he still has the responsibility to manage the behavior of his employees, including Rosenstein. Rosenstein has violated DOJ rules and ethics as well as threatening a sitting Congressman with subpoena power without probable cause. That is abuse of power.