"If it quacks like a duck and looks like a duck and walks like a duck, it's probably a duck. A fox can claim to be a duck all day long. But he's still a fox."

We've all heard that saying, or some version of it, a thousand times. The reason is simple: It's true. Our actions prove who we are. If a gulf exists between what we say, how we look and what we do, we're not living in a spirit of truth. A fox, even if he quacks, is still a fox. Sooner or later, it becomes obvious.

I remembered this last week as I read yet another news report about candidates who claim to be Catholic and then prominently ignore their own faith on matters of public policy. We've come a long way from John F. Kennedy, who merely locked his faith in the closet. Now we have Catholic senators who take pride in arguing for legislation that threatens and destroys life — and who then also take Communion.

The kindest explanation for this sort of behavior is that a lot of Catholic candidates don't know their own faith. And that's why, in a spirit of charity, the Holy See offered its guidance and encouragement in a little document last year On Some Questions Regarding the Participation of Catholics in Public Life.

Nothing in this Roman document is new. But it offers a vision of public service filled with common sense.

First, quoting John Paul II, it reminds us that, "man cannot be separated from God, nor politics from morality." In other words, unless our personal faith shapes our public choices and actions, it's just a pious delusion. Private faith, if it's genuine, always becomes public witness — including political witness.

Second, while Christians "must recognize the legitimacy of differing points of view about the organization of worldly affairs," they are also "called to reject, as injurious to democratic life, a conception of pluralism that reflects moral relativism." Appeals to a phony definition of pluralism and tolerance can never excuse inaction in the face of grave evil — including attacks on the sanctity of life. Catholics can only ensure real pluralism by "living and acting in conformity" with their religious convictions so that, "through political life, society will become more just and more consistent with the dignity of the human person."

Third, "(democracy) only succeeds to the extent that it is based on a correct understanding of the human person." Catholic lawmakers who do not vigorously seek to protect human dignity and the sanctity of human life from conception to natural death are not serving democracy. They are betraying it.

Fourth, "those who are directly involved in lawmaking bodies have a 'grave and clear obligation to oppose' any law that attacks human life. For them, as for every Catholic, it is impossible to promote such laws or to vote for them." Politics is the exercise of power. Power always has moral implications. And God will hold each of us accountable — from the average voter to senators and presidents — for how well we have used our political power to serve the common good and the human person.

"Pro-choice" candidates who claim to be Catholic bring all of us to a crossroads in this election year. Many Catholics, including some Church leaders, argue that "(we) should not limit (our) concern to one issue, no matter how fundamental that issue is." That's true — but it can also be misleading.

Catholics have a duty to work tirelessly for human dignity at every stage of life, and to demand the same of their lawmakers. But some issues are jugular. Some issues take priority. Abortion, immigration law, international trade policy, the death penalty and housing for the poor are all vitally important issues. But no amount of calculating can make them equal in gravity.

The right to life comes first. It precedes and undergirds every other social issue or group of issues. This is why Blessed John XXIII listed it as the first human right in his great encyclical on world peace, Pacem in Terris. And as the U.S. bishops stressed in their 1998 pastoral letter Living the Gospel of Life, the right to life is the foundation of every other right.

The humorist James Thurber once wrote that "you can fool too many of the people too much of the time." Our job as Catholics this election year — if we're serious about our faith — is to not get fooled.

Candidates who claim to be "Catholic" but who publicly ignore Catholic teaching about the sanctity of human life are offering a dishonest public witness. They may try to look Catholic and sound Catholic, but unless they act Catholic in their public service and political choices, they're really a very different kind of creature.

And real Catholics should vote accordingly.