The obligations one takes on for ones faith are one's own. You can't make that decision for anyone else. You may try to persuade others to adopt your position, but you're not required to. And in particular, a politician has to serve all the people he represents, even those of a different religion or opinion, as well as he can. There is a difference, as Canadian Prime Minister Pierre Trudeau said, between sin and crime.
Also, keep in mind that advocating abortion rights does not mean advocating abortion. It means allowing people to have free will, which I recall is something God thought was important. Most liberals would rather eliminate the need for abortions in the first place.