The liberal Washington-based pressure group, outraged at Vatican obstructionism at recent UN conferences on world population and on women, has launched a write-in campaign to the UN Secretary-General, Kofi Annan. The Vatican, it says, should be treated solely as a religious body. No longer should it be deemed a "Non-Member State Permanent Observer" - often with the voting rights - but as an ordinary non-governmental organisation, like the World Council of Churches and every other religion represented at the UN.
"We've sent out 150,000 cards," said Frances Kissling, head of Catholics for a Free Choice, "and 275 organisations have endorsed us. They include women's groups, abortion and family-planning groups - not only Christian but Muslim, too."
The response, she claimed, has been very good. "People love it, they thinks it's a great idea, it's something that clicks with them. One staff member went out to collect signatures from a queue for the latest Star Wars movie here in Washington and came back to the office with 100 signed cards."
To say the Vatican is trembling at the assault would be an exaggeration, and Ms Kissling is under few illusions. "I see this as David against Goliath. Rationally, we know we've got no chance whatsoever of early change, given the pace at which things move at the UN. On the other hand, part of being a person of faith is believing in miracles."
The Church's hostility is not in doubt. "They hate us - but there's not much they can do about it," said Ms Kissling. "These matters are not doctrinal and we are lay Catholics. And what's more, most Catholics agree with us."
In short, there is unlikely to be any problem with the gentlemen from the Congregation for the Doctrine of the Faith, better known under its old name of the Inquisition.
If nothing else, however, the campaign has refocused attention on the split historical personality of the Roman Church - spiritual beacon for a billion people around the globe but also an entity with more than a thousand years of temporal power coursing through its veins.
Ms Kissling maintains - and few would disagree - that the Vatican is a most peculiar institution. Its concept of citizenship extends little further than the few hundred people who live on its 108 acres. It has, famously, no divisions. But although the medieval Papal States have shrunk to an enclave in the heart of Rome, the Vatican City is none the less a state, boasting a government (among the most absolutist on earth) as well as diplomatic relations with more than 100 countries.
Which would be fine, says Catholics for a Free Choice, if the Vatican confined its temporal activities to purely political matters, such as undermining Communism or serving as a conduit or neutral mediator in international disputes. "The problem is that they want it both ways, as both a temporal and a spiritual power."
So it was at the UN conference on global population in Cairo, and last year at the UN conference on women in Peking. At a time when religious fundamentalism threatens pluralism, tolerance and rights, says the Washington group, "it's more important than ever that the UN maintains a clear separation between religious beliefs and international public policy".
Unabashedly, however, the Vatican uses the former as a battering ram, to try to shape the latter - above all on birth-control issues. The latest focus of liberal Catholic anger is its effort to prevent emergency abortions for women raped during the Kosovo conflict.