The fact that he's likely to finish in first doesn't mean he's going to win the nomination today, however. As David Nir writes in today's Daily Kos Elections Morning Digest, there will be a runoff election on April 2 unless Sanford or one of the 15 other Republicans manages to break 50 percent. That will almost certainly not happen, so even if Sanford does ultimately return to Congress, he's got at least two more elections ahead of him not including tonight's.
Whoever wins the April 2 runoff will face Elizabeth Colbert Busch, Stephen Colbert's sister, in the general election on May 7, 2013. Assuming Sanford does ultimately get the nomination, it will be a fun campaign to follow, if nothing else. Sure, Mitt Romney won the district by 18 points in 2012, so it's a pretty heavily Republican district, but just the thought of a campaign where you could conceivably run an ad morphing Mark Sanford, John Edwards, and Scott DesJarlais is enough to make me chuckle.