From the Archdiocese of Atlanta site:
As bishops, we have the obligation to teach and guide the Catholic Faithful whom we shepherd in the Body of Christ. A fundamental teaching of our Church is respect for the sacred gift of life. This teaching flows from the Natural Law and from Divine Revelation.
Life is a gift bestowed upon us by God, a truth underscored by the commandment: "You shall not kill" (Deut 5: 17 ). The Old Testament also teaches us that human life in the womb is precious to God: "...I formed you in the womb..." (Jer 1: 5). The right to life is a value "which no individual, no majority and no State can ever create, modify or destroy, but must only acknowledge, respect and promote" (Pope John Paul II, Evangelium vitae, 71a). A law, therefore, which legitimizes the direct killing of innocent human beings through abortion is intrinsically unjust, since it is directly opposed to the natural law, to God's revealed commandments, and to the consequent right of every individual to possess life, from the moment of conception to the moment of natural death.
Catholics in political life have the responsibility to exemplify in their public service this teaching of the Church, and to work for the protection of all innocent life. There can be no contradiction between the values bestowed by Baptism and the Catholic Faith, and the public expression of those values. Catholic public officials who consistently support abortion on demand are cooperating with evil in a public manner. By supporting pro-abortion legislation they participate in manifest grave sin, a condition which excludes them from admission to Holy Communion as long as they persist in the pro-abortion stance (cf. Canon 915).