In the Iowa caucuses, GOP turnout fell short of expectations. In the New Hampshire primary, it happened again. Turnout in South Carolina was strong, but after another weak showing in Florida, it's proving to be the exception. … At this point in the nominating process, the last thing party leaders wanted to see was evidence of a listless, uninspired party, underwhelmed by their field of candidates. Republicans probably won't fret publicly, but the turnout numbers should give party leaders pause.
Perhaps Michael McDonald's research on Florida can help explain the South Carolina exception:
I've plotted by county the percent vote for Gingrich against the percent change in turnout from 2008 to 2012. The graph tells a clear story. In counties where Gingrich did better, Republican turnout was up over 2008. In counties where Romney dominated, turnout was lower.