They're investigating crimes they think he has already committed. Doesn't matter whether he's still mayor or still running for mayor. They're looking for proof of what they believe already happened, and it doesn't matter whether he was mayor at the time or not (unless it involves influence peddling or other corrupt practice directly related to his position). Drugs, gangs, guns, murders, beatings, extortion -- that's what they're after. If he does drop out or is defeated, that removes the possibility that an arrest is a political act, making the optics easier on the TPS if they finally act. The only effect on Brazen2 is to potentially prevent gathering evidence of new crimes, I would say.