It’s not because of who lives there, writes Jordan Ragusa. It’s because of when they vote. South Carolina is usually the last real chance that second-tier candidates have to topple the front-runners, and so they have little choice but to pull out all the stops:
Because the primary system is an iterated process (rather than a one-shot, 50 state election), political “momentum” is critically important (see this paper by John Aldrich for a formal proof of this dynamic). Simply put, candidates who win early primaries like Iowa and New Hampshire are likely to receive greater support in subsequent states because of sophisticated or “front runner” voting (see this paper) as well as generate greater campaign donations and support. This, in turn, improves their chances of winning subsequent primaries. Because South Carolina is third in this sequence, there is an incentive for opposition candidates to go negative independent of the state’s demographics.