Based on how these discussions have gone in the past, and how they have often been resolved, what do we think of this as a solution?
Mandate that all campaign DMs (including forum PMs, in case people felt the distinction was particularly important) be publicly posted in the candidate's thread. In the past when this has flared up, people sharing the DMs they received often soothed feelings. It also forces the candidates to be very deliberate and judicious in the messaging they use in the DM, as they know it will be seen by a wider audience. And since this will be a known requirement, or one that people can observe, it's more likely that an errant DM that is not reported could be reported. Candidates falling out of compliance could then be subject to prosecution, or if we don't want to go that far, they take a hit in the court of public opinion for trying to pull a fast one on their opponent. This is also commonplace in WA voting - campaign TGs often get shared in the discussion thread on the NS forum, particularly when there's an effort for that campaign to exclude certain other players.
Naturally this will lead some to wonder about what constitutes "campaign." If my approach is desirable, this is where I imagine the bulk of the debate will center. I would propose that any DM that directs players to view campaign threads or the voting thread, or provides links to these areas; explicitly asks for a vote or for a vote to be changed; provides a narrative or argument for why the voter should choose this candidate or not choose the other (think campaign mail or flyers that give a quick pitch for or against a candidate); or was unsolicited by the player would constitute mandated reporting. Because these messages can often be tailored to specific individuals, I would further specify that every unique DM would need to be reported separately. It may be the case a candidate is sending the same message to everyone they reach out to, but often they vary, and the variations can matter. Furthermore, we could extend these requirements to surrogates as well using the same conditions, as that's obviously a way around this.
If a voter comes to the candidate asking for questions or clarification, I see no reason that needs to be reported - for one thing, the voter is directing that process, and inviting the arguments and comparison, and may have a reason for not pursuing something similar from the other candidate. People seeking knowledge and answers to help them make their decision is part of the process and I would err on the side of looking after their privacy rights. By a similar token, once the pitch has been made and the conversation happens, I am not suggesting we mandate reporting of the entire conversation. I can understand how the back and forth may give the candidate a chance to go from an innocent sounding message to dark conspiracy blatant falsehoods, but aside from it being a huge risk for the candidate to take in the event the other party shares the exchange, the transparency surrounding it provides a contrast not only for that voter but for others who are receiving a similar message. If all the public DMs are nice and polite and milquetoast, the seedier darker stuff will stand out more and makes it more likely it gets revealed.
I do not see this as violative of free speech - the candidate is still free to send DMs, there's just certain rules of conduct for doing so.