The Republican Stance on Abortion: A Political Analysis

Question
I'm more against than for abortion however I'd like your opinion on why this is such a huge Republican platform.  It's my understanding that abortion's been legal for over 30 years.  In those years 5 of those 7 presidents have been Republican and the legality of abortion's never changed.  Why do so many vote for a president based solely on one issue which isn't even an issue?  

Answer
A.J.
 The smart-ass, cop-out response is 'that's politics. Sadly, it's also mostly true. Issues change parties, and parties change priorities. Also, a republican president will have no chance changing a law when congress is ruled by democrates. Also, many people believe that abortion is a big enough issue that it breaks party lines, therefore making it even harder for any laws to be passed. Ok, so yeah, knowing all of this I still vote republican, even when the majority of my other beliefs are typically democratic, only because the best chance for a reverse of the abortion situation at this time is a republican president.
It's all very fuzzy, but then again. . . that's politics!
Tara

P.S.  I hope that helped!