Well typically a case is referred to them by law enforcement or a criminal complaint, then they review the evidence and decide whether or not to go to trial.
It's much rarer for a DA to say they want to find crimes a particular person committed and then direct others to go find evidence for whatever they can find evidence for.
Which DA went after a particular person without referral by law enforcement or a criminal complaint? If this was really such a breach of ethics, surely it would be trivial for the political party in question to first make a criminal complaint? It's not obvious to me that the complaint/referral matters much?
It's much rarer for a DA to say they want to find crimes a particular person committed and then direct others to go find evidence for whatever they can find evidence for.