“Those who follow him [Jesus Christ] live in the Spirit of truth and guard against duplicity, dissimulation, and hypocrisy.”
–Catechism of the Catholic Church, section on the Commandment against bearing false witness
Given the dishonesty pervading the campaign, I thought it might be useful to review why lying was included in the top ten commandments.
Here are excerpts from the Catechism of the Catholic Church. (Note that in Catholic translation’s the prohibition on bearing false witness is the Eighth Commandment. In many Protestant and Jewish translations, it’s the Ninth Commandment.) For a Christian, falsely defaming another person’s character or reputation violate Christ’s commands.

521. What is one’s duty toward the truth?
Every person is called to sincerity and truthfulness in acting and speaking. Everyone has the duty to seek the truth, to adhere to it and to order one’s whole life in accordance with its demands. In Jesus Christ the whole of God’s truth has been made manifest. He is “the truth”. Those who follow him live in the Spirit of truth and guard against duplicity, dissimulation, and hypocrisy.

522. How does one bear witness to the truth?
A Christian must bear witness to the truth of the Gospel in every field of his activity, both public and private, and also if necessary, with the sacrifice of his very life. Martyrdom is the supreme witness given to the truth of the faith.
523. What is forbidden by the eighth commandment?
The eighth commandment forbids:
• false witness, perjury, and lying, the gravity of which is measured by the truth it deforms, the circumstances, the intentions of the one who lies, and the harm suffered by its victims;
• rash judgment, slander, defamation and calumny which diminish or destroy the good reputation and honor to which every person has a right;
• flattery, adulation, or complaisance, especially if directed to serious sins or toward the achievement of illicit advantages.
A sin committed against truth demands reparation if it has caused harm to others.
524. What is required by the eighth commandment?
The eighth commandment requires respect for the truth accompanied by the discretion of charity in the field of communication and the imparting of information, where the personal and common good, the protection of privacy and the danger of scandal must all be taken into account; in respecting professional secrets which must be kept, save in exceptional cases for grave and proportionate reasons; and also in respecting confidences given under the seal of secrecy.
525. How is one to use the means of social communication?
The information provided by the media must be at the service of the common good. Its content must be true and – within the limits of justice and charity – also complete. Furthermore, information must be communicated honestly and properly with scrupulous respect for moral laws and the legitimate rights and dignity of the person.