Let's say Alice and Bob are candidates running against each other for political office.
Let's say Alice's campaign gets hold of a video of Bob, taken in a restaurant during a foreign vacation some years earlier, in which Bob says, "I have to say, I do like the taste of this horse meat."
And let's say her campaign rightly believes that an ad featuring this video would repay itself in votes for Alice from the anti-horse-eating demographic.
Would it be wrong for Alice's campaign to run such an ad?
If the role of public authority is to ensure the common good (see CCC 1898), then the role of a political campaign is to put forth reasons why its candidate would best ensure the common good.
To intentionally put forth invalid reasons is to lie, and to lie is always wrong.
If, therefore, Alice's campaign doesn't believe the fact that Bob once ate horse meat is a valid reason (or at least further evidence) Alice would better ensure the common good, then it would be wrong for them to run a commercial implying so.
If they do believe it's a valid reason or further evidence Alice is the better choice, then I think they could run an ad, as long as it gives the reason or explains the evidence.