Another favourite of theirs is to say "my conscience tells me it's right and the Church teaches we are answerable to our consciences". I've heard Ms Pelosi go down this route.
Complete rubbish of course and further proof of their ignorance of what the Church actually teaches. The Catholic Catechism says a conscience should be PROPERLY FORMED for one to make decisions based on their "conscience".
If a politician doesn't even know the BASICS (i.e. that abortion is intinsically evil and always wrong thus making the pro-choice stance incompatible with the Catholic one) then obviously their conscience is NOT properly formed and any moral judgements they make according to their "conscience" is flawed.
http://ncronline.org/NCR_Online/archives/012403/012403a.htm
“I didn’t think I wanted to be a nun,” the new House minority leader, Nancy Pelosi, recalled. “But I thought I might want to be a priest. There seemed to be a little more power there, a little more discretion over what was going on in the parish.”