Just think about it: Pro Life vs Pro Choice, when framed this way the morally right stance is obvious. Being "Pro life" implies that you are anti death, the obvious moral high ground. By being the opposite of Pro life, being Pro choice implies being pro death, or at least for being able to choose death.
I don't know which expression came first, (I would guess Pro Life in response to Roe v Wade) but either way, Democrats did a terrible job at expressing their values.
I believe most Liberals support a woman's right to have an abortion because they believe that a woman cannot be free, unless she has complete control over her own body and the authority to decide when she becomes a mother. Therefore liberals would have been much wiser to frame their perspective on the issue as being "Pro Liberty". America was founded on the idea that liberty is essential to life. The concepts share a moral high ground, and the term more accurately reflects how people feel about the issue.
On a similar note Democrats should have never endorsed the term abortion. The term implies bailing from an intended course of action. Most woman choose to have abortions because they never intended to get pregnant in the first place, not because they changed their mind. Democrats would have been wise to refer to the procedure as "development prevention" as in preventing an embryo from developing into a fetus, or a fetus into a human.
I got these ideas about framing from George Lackoff, the author of The Little Blue Book: The Essential Guide to Thinking and Talking Democratic
He argues that Democrats frequently shoot themselves in the foot by adopting conservative framing of issues, or just repeating them, while arguing against them. That people make decisions based on their moral values, and that Progressive and Conservative moral values both exist, but Democrats are bad at appealing to progressive moral values, and instead cite facts.
If you are a Liberal/Progressive, this is a must read. If you are a conservative I think this would be a very interesting read. He makes some claims about Conservative values that I would be interested to hear if most conservatives agree with, and I think it gives a good insight at why Democrats take the stances on issues that they do.
Its just over 100 pages and only ten bucks.