If I'm not mistaken, SC almost voted for Ike twice and barely voted for Carter. When you consider that it had Strom Thurmond who flipped to the GOP well before Republicans had power in the South (say what you want about civil rights, there were MANY more segregationist Democrats who remained in the party until the day they died), it seems they've always been more conservative than their neighbors (or at least post-New Deal).
Jimmy Carter actually received over 56% of the vote in South Carolina in 1976 and Ronald Reagan barely won it in 1980.