We have an adversarial legal system. Prosecutors are supposed to be biased towards conviction and defense attorneys are supposed to be biased towards acquittal. Judges are to be fair.
Political bias won't make a prosecutor try harder to get a conviction. It might affect the decision to investigate or charge someone in the first place but that's not the prosecutor's call.
Do you really believe that the Dem hacks in the DOJ won’t try harder to get a visible Republican scalp than going after a Dem? Oh wait, Dem cases are simply dropped by the Dem hacks in the DOJ. No worries.