I don’t think that’s exactly true. There are certainly a large number of people that will show up and vote against Obama even if its revealed Romeny bathes in fetus blood every night to preserve his complexion. And as the most committed voters, those people are generally the most visible so its easy to think they make up most of the people that will vote for Romney. But I don’t think they make up 50% of the electorate, which is what Romney actually needs to show up and pull the lever for him.
Indeed, appealing to those people that aren’t slavering at the mouth to get rid of Obama is pretty much the whole reason Romney is the GOP nominee instead of someone more like Bachman, Newt, Cain, etc.
He (supposedly) was always prolife in his personal life, but supported Roe v Wade until 2007 or so.