"The inviolability of the person, which is a reflection of the absolute inviolability of God, finds its primary and fundamental expression in the inviolability of human life. Above all, the common outcry, which is justly made on behalf of human rights -- for example, the right to health, to home, to work, to family, to culture -- is false and illusory if the right to life, the most basic and fundamental right and the condition of all other personal rights, is not defended with maximum determination . . . " (From John Paul II's 1988 apostolic exhortation, The Vocation and the Mission of the Lay Faithful in the Church and in the World (Christifideles Laici))

or this one,

"But being 'right' in such matters [policies on poverty, employment, education, etc.] can never excuse a wrong choice regarding direct attacks on innocent human life. Indeed, the failure to protect and defend life in its most vulnerable stages renders suspect any claims to the 'rightness' of positions in other matters affecting the poorest and least powerful of the human community." (From the US Bishops, Living the Gospel of Life, 1998)

or this one,

Not all moral issues have the same moral weight as abortion and euthanasia. Ratzinger memo

or this one,

"The Church recognizes that there are occasions when war and the death penalty are justified, even though such measures are undesirable and should be kept to the necessary minimum . . . Catholics who fully accept the doctrine of the Church can sometimes disagree about whether a given war or death sentence is morally defensible...Abortion is in a different class. As the deliberate taking of innocent human life, direct abortion can never be justified. About the moral principle, there can be no debate in the Church. The teaching has been constant and emphatic." Cardinal Avery Dulles

or this one,

Since abortion and euthanasia have been defined by the Church as the most serious sins prevalent in our society, what kind of reasons could possibly be considered proportionate enough to justify a Catholic voting for a candidate who is known to be pro-abortion? None of the reasons commonly suggested could even begin to be proportionate enough to justify a Catholic voting for such a candidate. Reasons such as the candidate’s position on war, or taxes, or the death penalty, or immigration, or a national health plan, or Social Security, or AIDS, or homosexuality, or marriage, or any similar burning societal issues of our time are simply lacking in proportionality. Bishop Rene Henry Gracida

or this one,

Certainly policies on welfare, national security, the war in Iraq, Social Security or taxes, taken singly or in any combination, do not provide a proportionate reason to vote for a pro-abortion candidate.

Consider, for example, the war in Iraq. Although Pope John Paul II pleaded for an alternative to the use of military force to meet the threat posed by Saddam Hussein, he did not bind the conscience of Catholics to agree with his judgment on the matter, nor did he say that it would be morally wrong for Catholic soldiers to participate in the war. In line with the teaching of the catechism on "just war," he recognized that a final judgment of prudence as to the necessity of military force rests with statesmen, not with ecclesiastical leaders. Catholics may, in good conscience, support the use of force in Iraq or oppose it.

Abortion and embryo-destructive research are different. They are intrinsic and grave evils; no Catholic may legitimately support them. In the context of contemporary American social life, abortion and embryo-destructive research are disproportionate evils. They are the gravest human rights abuses of our domestic politics and what slavery was to the time of Lincoln. Catholics are called by the Gospel of Life to protect the victims of these human rights abuses. They may not legitimately abandon the victims by supporting those who would further their victimization. Archbishop John Myers

or this one,

"If the reasons are really proportionate, and the person remains clear about his or her opposition to abortion, that can be done. What is a proportionate reason to justify favoring the taking of an innocent, defenseless human life? That's the question that has to be answered in your conscience. What is the proportionate reason?" He said that the difficulty of these nuanced ideas is the reason he did not discuss the problem of proportionality in his earlier statements in June. "It is difficult to imagine what that proportionate reason would be,"  Archbishop Raymond Burke

or this one,

More to the point, however, it is a classic abuse of religion that people will invoke a religious authority to justify violence. There is no act of violence more brutal, or claiming more victims than abortion. It kills 4000 children daily in the United States alone. Practitioners admit under oath that they dismember and decapitate these babies. If people try to equate that with other issues by misquoting a Vatican Cardinal, they are guilty of the most shameless type of abuse of religion -- the same kind of abuse used to justify burning people at the stake. Only this time, we're talking about babies."
Father Frank Pavone