There is nothing worse than abortion. The slaying of infants is unspeakably evil. Thus,
“The inalienable right to life of every innocent human individual is a constitutive element of a civil society and its legislation:
“‘The inalienable rights of the person must be recognized and respected by civil society and the political authority. These human rights depend neither on single individuals nor on parents; nor do they represent a concession made by society and the state; they belong to human nature and are inherent in the person by virtue of the creative act from which the person took his origin. Among such fundamental rights one should mention in this regard every human being's right to life and physical integrity from the moment of conception until death.’
“‘The moment a positive law deprives a category of human beings of the protection which civil legislation ought to accord them, the state is denying the equality of all before the law. When the state does not place its power at the service of the rights of each citizen, and in particular of the more vulnerable, the very foundations of a state based on law are undermined....As a consequence of the respect and protection which must be ensured for the unborn child from the moment of conception, the law must provide appropriate penal sanctions for every deliberate violation of the child's rights.’” (Catechism of the Catholic Church §2273) 
Of the two major political parties in the United States, the Republican Party is friendly to this view , while the Democratic Party is decidedly unfriendly.  Because of this, many Catholics have decided to throw in with the Republicans. To the objection that the Republican Party has taken positions that are clearly contrary to Catholic teaching in areas such as labor rights, the social safety net, and the environment, the response has been that abortion, since it has resulted in the taking of millions of lives, is by far the most important issue, overshadowing every other consideration.
Such reasoning is sound, as far as it goes. But too often it is used as a means of disassociating one’s self from the evil effects a vote cast for a Republican might bring about. No Catholic voter should simply shrug at such governmental actions as cutting food stamps or the prosecution of an unjust war. Indeed, the cutting of food stamps might well result in the death of innocents, and an unjust war will most certainly do so. Every human life is of infinite value, not just the lives of the unborn.
This is not to say that Catholics should avoid voting for Republicans, or that they should not become Republicans. But in today’s American political environment, Catholics should not feel free to be so passive as to receive satisfaction on one issue, even if it is the most important issue, and wink at the rest. Whichever of the two major political parties Catholics choose to align with, they have a duty to try and transform it so that it truly serves the common good. Even in politics we are called to be the salt of the earth, the light of the world. Thus, just as surely as Catholic Democrats must work to transform their party into one that is pro-life, Catholic Republicans have a duty to make every effort to make their party friendlier to the cause of labor and the needs of the poor.
It really is the least we can do.