While Catholic Tradition is often portrayed as being a sort of separate, and higher authority than the Bible, it is actually used in order to interpret the Bible. What do we do when the Bible does not explicitly state "abortion is wrong" or "life begins at conception, which is at fertilization and not implantation"? For the Catholic Christian, turning to Tradition can be a powerful tool in showing the Church's unchanging stance on this issue. Despite what some prominent politicians might say.
The not-quite-Catholic blogger at Catholidoxy has compiled a long list of affirmations on the Church's stance on abortion from our Fathers in the Church.
It's important to note that the Jewish and Christian attitudes toward pregnancy, contraception (it existed in the ancient world), abortion, infanticide, marriage, family and sex conflicted with Greco-Roman perspectives. Abortion and infanticide were common (remember the Spartan baby inspector in the movie 300 debating whether or not to throw the baby Leonidas down the cliff with the other discarded babies?). It is a great triumph of Christianity that we stamped these practices out of western society.
Until politicians like Nancy Pelosi came along, that is, mistaught by miscreant mediocre theologians who wanted to overthrow the tradition of the Church (Fr. Drinan comes to mind). On this issue and so many others, the West is slipping back into the brutality of paganism.