There is head-scratching in the political marketplace over a looming contradiction. The candidates for president of the United States include a man identified as a Roman Catholic, and among the voters there are, of course, many Catholics. It would be reasonable to suppose that Candidate Giuliani would get the presumptive backing of the Catholic population.

But there are a couple of caveats.

First, is Giuliani a Catholic other than nominally? Because his name is Italian, one assumes that he subscribes to the faith associated with the Italian people. As a boy, he went to Catholic schools, and he was apparently devout; he even contemplated entering seminary. But is he a practicing Catholic now?

In 1999 the question of his religious faith was put to him directly. His reply: "I don't attend (Catholic services) regularly, but I attend occasionally."

Now, plop! this raises special problems in the Catholic communion. Catholics are not only expected to attend Mass every week, they are bound to do so. In the matter of the Sabbath, you can be an easygoing Episcopalian, or Quaker, or even Reform Jew, and no rule is broken of formal consequences. But that isn't so in the Catholic communion, because there are rules that include attendance at Mass on Sundays. If you're a Philadelphia lawyer you might here smile a bit and say, well ... Christians don't always behave as Christians, so what else is new?

Ah, but that doesn't work. Because the kind of godlessness expressed by a failure to live a life of charity, sustained by faith and hope, is, unhappily, pretty unnoticeable. Everyone excepting the saints is, under such scrutiny, "un-Christian." But a failure to attend church on Sunday is, by Catholic standards, contumacious, an ostentatious rejection of a formal obligation. It is the equivalent of an observant Jew biting into a piece of pork. Penitence, if genuine, can minister to any infraction of the faith. But to violate systematically the commandment that says, "Remember the Sabbath and keep it holy," is systematically to reject one's faith.

There are other problems in the matter of Mr. Giuliani. One's sense of things is that the religious communities are understanding in the matter of failed marriages (the divorce rate in the United States has been estimated at about 50 percent), but those who aspire to lead are quite reasonably examined more closely, and in the matter of Giuliani, there is the second and then the third wife, with ugly consequences involving children and living quarters.

Which is to say that a candidate holding out his affiliation with a religious body as a reason to presume harmonious values with other voters of the same faith has to prepare for a likelihood of resentment among coreligionists if he appears lax in the practice of his faith. Members of a club can be relaxed about the member who does not pay his dues. But there is the risk there of continued neglect gradually understood as disloyalty.

The way things work in modern times, under modern pressures, more people's attention is attracted by defiance of a protocol than by inconsistent attention given to it. The guest who neglectfully fails to bow when the queen enters the room is not especially conspicuous, but becomes so if it crosses the mind of others that he is challenging the legitimacy of the sovereign, rather than merely to being absent-minded about protocols.

There is the factor that in any political contest others are aspiring to win the voters' approval. It is natural that candidates will call attention to the failures of their rivals, and that interested observers will join in. Gary Bauer, for instance, a longtime champion of the relevance of the Christian faith in politics, cannot be expected to be indifferent to the anomalies we speak of. James Dobson is likely to be heard from. And then there are the bishops and priests who will not wish to be thought indifferent to the indifference of others to the cosmic commitments they have made.