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Speaking as a Catholic, I feel that there is a very real danger for the church if it were to allow pro-abortion Catholics in public life to go unchallenged. Such inaction would imply that opposition to legalised abortion and other anti-life legislation is not terribly important, and that it's more important for the church to be, as it were, a respectable part of the "right to choose" state - expressing opposition to abortion, certainly, but not in such a way as to upset the apple cart. In this way the church's teaching, as expressed by Pope John Paul II in Evangelium Vitae (73) below, would become obscured:
"Abortion and euthanasia are thus crimes which no human law can claim to legitimize ... there is a grave obligation to oppose them by conscientious objection ... In the case of an intrinsically unjust law, such as a law permitting abortion or euthanasia, it is therefore never licit to obey it, or to 'take part in a propaganda campaign in favour of such a law, or vote for it."