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Prior to Reagan in 1980, the GOP wasn't really much of a Pro-life party. Jimmy Carter after all was the first born-again president and George HW Bush was pro-choice before he ran for president. Even Reagan didn't make a big deal out of it in 1976 IIRC.
What if the GOP hadn't become the pro-life party? Being pro-life brought a lot of single-issue voters (Catholics in particular) into the GOP who otherwise aren't all that conservative.