It’s probably the other way around, ie he began a relationship with his staffer, rather than making a previous mistress a staffer.
If it’s the latter and that can be proven, that might be an effective issue. Calling him adulterous scum for cheating on a wife that doesn’t remember him anymore won’t help though, at least I don’t think so.
I think that Cochran’s personal behavior is fair game, since he “opened the door” when he accused McDaniel of being involved in the “break-in” of his wife’s hospice. While not everyone whose wife suffers from Alzheimer’s will remain as loyal as James Garner’s character in The Notebook, Cochran’s behavior is unseemly. And either Cochran pays his executive assistants better than anyone in Washington, or that $1.7 million home in DC was bought by Cochran in his mistress’s name (or maybe his mistress is independently wealthy and owns the home for real, in which case it would make it far likelier that she was his mistress first and took on the executive-assistant position so as to be able to travel with Cochran without raising suspicion, all at taxpayer expense).