Is it moral to broadcast the sins of a candidate for public office?

The Catholic Encyclopedia teaches that you can broadcast “the official shortcomings of public men.” And you can “present whatever information about the life or character of a candidate for public office is necessary to show his unfitness for the station he seeks.”

The Catechism itself even says that private information that negatively affects someone may be divulged if there is "a grave and proportionate reason."  

Here is the full text of the Catechism: "Professional secrets - for example, those of political office holders, soldiers, physicians, and lawyers - or confidential information given under the seal of secrecy must be kept, save in exceptional cases where keeping the secret is bound to cause very grave harm to the one who confided it, to the one who received it or to a third party, and where the very grave harm can be avoided only by divulging the truth. Even if not confided under the seal of secrecy, private information prejudicial to another is not to be divulged without a grave and proportionate reason."

Still it also emphasizes that the content of communication should "be true and--within the limits set by justice and charity--complete. Moreover, it should be communicated honestly and properly. This means that in the gathering and in the publication of news, the moral law and the legitimate rights and dignity of man should be upheld." 

In summary, you can talk about the faults of a candidate for public office as long as you do so with truth, justice, charity, propriety, and in the service of the common good.