I guess I should expand on what I said now that there's been an argument against the proposal, especially since I've expressed essentially the same position as nyttyn in discussions on IRC before.
I agree that a ban on campaigning is essentially unenforceable and a feel-good law, and I expected to oppose any proposal to make an actual rule against campaigning. However, I sympathize with paintseagull's position as well. If we see something we consider wrong and against CAP's philosophy, the reasonable response should be to make a rule against it. (God knows CAP doesn't have a problem with making rules against things.) My issue with a rule, though, was always that in the bulk of circumstances, we can't really enforce it. We can never really be sure that someone on another site with the same username as a CAP regular is actually the same person, and it is not fair to punish someone if we don't have actual proof that they did it. As such, my preference had been to go no further than a "we would really prefer if you didn't try to campaign"-type statement.
However, I think paintseagull's proposal is a little big better. In practice, it usually won't amount to much more than a more strongly-worded version of such a statement, since it leaves any punishment up to the mods' discretion. (I would hope and expect that the mods would be lenient when they aren't entirely certain that it really was the accused who did it.) However, I think there are two major advantages to having such a rule over simply "disapproving" of campaigning and leaving it at that.
First of all, it encourages people who encounter campaigning to report it to the mods. There are a couple of reasons I think this is good. One is that it means that the mods can keep track of any incidents that happen. They can speak to the accused and get the other side of the story. It's probably a good thing that the mods are at least aware of campaigning going on, even if they can't do much about it. Another upside is that if people feel that they have "done something" by reporting campaigning that they see to the mods, they may be deterred from doing something like the name-and-shame on IRC we saw during CAP 5. (Explicitly saying that doing so is against the rules is good, too.)
The other upside to having a rule like that relates to this bit that Pwnemon said on IRC:
Jul 07 13:46:38 <Pwnemon> if someone rigs a cap vote and then comes on #cap and brags about what they just did and we have explicitly said what they can do is legal
Jul 07 13:46:39 <Pwnemon> we're tied
Now, I don't really feel like that is especially likely to happen. However, in those rare situations where we really can be completely certain that someone is campaigning, it's good if we can do something about it. I stress that I really don't expect this to happen very often, if it ever happens at all. But maybe we can sleep better at night knowing that if someone really wants to be an idiot and brag about it, the mods can actually do something about it?
I do agree that this rule would be mostly toothless. However, all things considered, I think that it would do some good to have it anyway. Maybe it would be best to indicate in the actual wording that punishment won't always happen.