They don't, is just variations in the sample. Abortion has been pretty much 50/50, since like forever.
It's also one of the easiest positions to understand - I get it. They think that zygotes - even sperm in the catholic case are viable real human beings. If you put yourself in that state of mind - the passion and horror are quite understandable - which is why expressions like "pro life" and "late term /partial birth" etc are so powerful because they make a broad and strong case that all the biology is a person.
There are genuinely caring, emotionally invested people on the anti abortion side that are bought into semantics - the abortion battle has often been fought in the thesaurus rather than the courts of law or public opinion.
And the measuring stick for when a clump of cells becomes a person is not only a stick of differing length depending which faction is holding it, that stick is stymied by a lack of an easy, clear scientific consensus because the stages are subjective and analog. If a baby became a baby suddenly at a predictable stage and the change was binary and obvious, it might be an easier thing to compromise on.
Many anti abortion folks probably sit closer to the pro abortion people on the issue of when - almost zero abortion advocates want embryos with beating hearts and brains aborted - but further, in a secular democracy the people are free to get moral guidance from the church - but not law and not science. Those two things are secular in the US. And the GOP stance on abortion is hypocritical and intellectually dishonest. The GOP on abortion is a bus that drops you off miles from the Roe Vs Wade stop on purpose, because without the specter of that destination - they lose a significant amount of the evangelical and religious base. This time they might have to get closer to that stop than the party rationally desires. Deal with the devil.
