Santorum is uniquely loathsome because he 1) presents himself as The One True Catholic in the race while 2) advocating positions repugnant to Church teachings and 3) de-legitimizing the right of anyone who disagrees with him to identify as Catholic.
It might surprise many to find out that Santorum does not walk in lock step with the Catholic Church on every issue. Juan Cole highlighted ten areas where Santorum unambiguously breaks with official Church doctrine. The biggest break is Santorum's vigorous support for the Iraq war, which John Paul II and Benedict XVI emphatically opposed.
Andrew Sullivan also called out Santorum for his defense of torture:
In that very defense - in Santorum's own description of what he is defending - he is defending the "breaking" of a human person, made in the image of God. He is defending a core, absolute evil. Let us concede for the sake of argument that these are "enhanced interrogation techniques" and not "torture", as Santorum insists. There is no meaningful difference between the two whatsoever from a Catholic perspective, and Santorum's public positioning as an avowedly Catholic politician, while defending and promoting an absolute evil, is a true and immense moral scandal - in the Church's sense of the word. No one should be giving the impression that the Catholic church defends "enhanced interrogation techniques". This is from the Catechism:
"Torture which uses physical or moral violence to extract confessions, punish the guilty, frighten opponents, or satisfy hatred is contrary to respect for the person and for human dignity...Non-combatants, wounded soldiers, and prisoners must be respected and treated humanely. Actions deliberately contrary to the law of nations and to its universal principles are crimes, as are the orders that command such actions"
Notice there is a bar even on "moral violence" on or "frightening" prisoners. Santorum's own moral distinction between "breaking" human beings by EITs and "torture" does not exist in international law or Catholic doctrine.
These are fundamental issues where Santorum disagrees with the Church. There is nothing inherently wrong with forming an opinion in opposition to Church teachings. My conscience dictates that I disagree on a number of issues, but I am honest and upfront about it.
By presenting himself as a "Real Catholic," Santorum implies we must agree with all of his positions in order to rightfully call ourselves Catholic. As he infamously said, people who disagree with his vision of a theocratic America make him vomit.
It's no surprise Santorum carried the evangelical vote in Ohio and Michigan. His absolutist tone and conflation of GOP orthodoxy with religious values is far more typical of evangelicalism than it is of Catholicism.
What worries me is that as Church hardliners like Santorum and many bishops focus on sexual morality at the expense of everything else, the wall between reactionary evangelicalism and Catholicism is being eroded. With it comes the implicit dictate: the only way to be TRULY religious is to be a Republican. Even when being a Republican contradicts your religion.
"Conservative" and "liberal" are divisions we make here on earth. They are imperfect ways of categorizing the world and both fail to capture the totality of God's will and commandments. One does not need to be a Republican to be a Catholic. One also does not need to support Rick Santorum to be a Catholic. Luckily, Catholic voters seem to have this figured out.