It would be wrong to conclude that voters did not punish Mr. Sanford at all for his extramarital affair. In fact, a reasonable number of voters did appear to hold it against him. Last November, Mitt Romney won South Carolina’s First District by 18 percentage points. Since Mr. Romney lost the election to Barack Obama by roughly four percentage points nationwide, that means the First District is about 22 percentage points more Republican than the country as a whole.
That's an interesting way of looking at how partisan a district is as a whole, but it is a reasonable benchmark with applicable data germane to the district in question. Charlie Cook's PVI number for the district is R+11, so by that math, Sanford's indiscretions cost him next to nothing since Sanford won by 9, just a 2-point hit. Nate has other evidence to back up that 13-point figure, as he always does.
Mr. Sanford defeated his Democratic opponent, Elizabeth Colbert Busch, by nine percentage points instead – so one quick-and-dirty estimate is that Mr. Sanford’s personal history cost him a net of 13 percentage points. It just was not enough to flip the election result in such a conservative district.
As it happens, this 13-percentage-point penalty almost exactly matches an academic analysis on how much voters hold sex scandals against candidates. A 2011 paper by Nicholas Chad Long of St. Edward’s University, which examined United States senators running for re-election from 1974 to 2008, estimated that scandals involving immoral behavior lowered the share of the vote going to the incumbent by 6.5 percentage points.
Which results in a total 13-point swing if there are only 2 candidates in the race (and none of the 6.5 points are going to a third party.) That makes the case that Nate is correct on the impact.
Either way, the reason Sanford won was because he ran in an overwhelmingly Republican district created by gerrymandering and Tuesday night proved there are plenty of Republican voters willing to overlook any scandal to avoid voting for a Democrat, a far worse crime in their eyes than a question of character.
I can't think of a better example of what pundits mean by a "safe" district for a party: there's basically no way the seat will change hands. Keep in mind that for the vast majority of House seats, 80-85% of them, are this way for a reason (and yes, both parties do gerrymander, but only the party in charge in each state after the Census gets the benefit, which is why 2010 will hurt us for the next 4 House elections in 30+ states.)
Until more than 10% of House races are competitive, nothing will change. Even in a "wave" election like 2010, the GOP picked up 63 seats, only about 15%. But that's enough to all but assure they'll keep the House for the foreseeable future.
Oh, and long overdue tag: Keep Calm And Trust Nate Silver.