Any faithful Catholic running for the U.S. presidency is probably not electable. Liberals will not vote for him because he would be against abortion and against gay marriage.
A faithful Catholic can have liberal views on a number of issues, such as illegal immigration, the economy, the death penalty, foreign policy, etc. But certain social issues have become more and more important in politics, so that no matter how liberal a Catholic politician is on other issues, he would still be considered a conservative because of family values issues (as they call it).
Then, too, quite a few conservatives are only partially pro-life, and many have no reservations about gay marriage or gays in the military. So a devout Catholic politician will lose many conservative votes by being ‘too pro-life’ (politically speaking) and by being (supposedly) ‘anti-gay’.
Then the final nail in the political coffin is the anti-Catholic sentiment that prevails in the U.S. A strong anti-Catholic sentiment has been building as an undercurrent for some time. But at this point, there are enough persons with that sentiment that it has become socially acceptable.
Persons who do not devoutly practice any religion, even if they self-identify as a member of a religion, are basically living secular lives. They believe whatever the prevailing view is in sinful secular society, or within their particular socio-political group. They do not base their beliefs or their actions on religious teachings. These secular persons oppose Catholicism for many reasons: partly because the Church condemns sins that have become widely accepted in society (sexual sins, abortion, contraception), and partly because the Church teaches that people should believe and live in a certain way. Secular persons tend to resent any person or group telling them what they should believe or do. So an anti-Catholic sentiment pervades many persons who live secular lives.
Some non-Catholic believers oppose Catholicism because the Church’s teachings are contrary to their own devout beliefs. But this reason for anti-Catholic sentiment is becoming less and less common. It also tends to be fairly mild, being ameliorated by true faith and a sincere conscience.
More often a Protestant will oppose Catholicism for secular reasons, because Church teaching is contrary to their secular lives and their secular beliefs. Although a majority of persons in the U.S. self-identify as Christian, most do not base their beliefs and practices on Christian teaching. Rather, they adopt whatever is the prevailing view in sinful secular society, and then they either ignore Christian teaching, or they find some way to modify Christianity to make it teach exactly what secular society teaches. But when they are confronted by the definitive teachings of the Catholic Church — based on the three pillars of truth: Tradition, Scripture, Magisterium — they respond with resentment or anger or contempt.
What is most surprising and alarming is that many persons who call themselves Catholic have contempt, anger, even hatred for the teachings of the Church, esp. on sexuality and gender (abortion, contraception, homosexuality, sex outside of marriage, women’s ordination). There is literally a strong current (no longer an undercurrent) of Catholics who have anti-Catholic bias. They are angry at the teachings of the Church on many important issues, and they direct that anger also at any Catholic who believes and lives by those teachings.
As a result, many Catholics will not vote for a Catholic politician who believes and lives according to the Church’s teaching. So a devout Catholic politician loses many votes on the left as well as quite a few votes on the right. And he also loses many votes from Protestants and from his fellow Catholics. As this trend continues, it becomes more and more difficult, if not impossible, for a devout Catholic to be elected U.S. President. It may reach a point, at some time in the near or distant future, when a devout practicing Catholic is not electable to any major office.