While less striking than the historic Republican victory in New York City, the GOP victory in Nevada’s 2nd Congressional District is also worth mentioning, especially given the wide margin of victory. Nate Silver does the math:
One crude way to forecast the results you might expect to see out of a House race is through its Partisan Voting Index, or P.V.I., a measure of how the district voted relative to others in the past two presidential elections.
The Nevada Second, for instance, has a P.V.I. of Republican plus-5, meaning that the Republican candidate would be expected to perform 5 points better there than a Republican might nationally. Since a vote for the Republican is (usually) a vote against the Democrat, you need to double that number to project the margin of victory. In this case, that would imply a Republican win by 10 points given average candidates and a neutral overall political environment.
The Republican Mark Amodei, however, leads by 22 points as of this writing, an easy victory, meaning that he overperformed the P.V.I. by 12 points.
Silver goes on to conclude, based on average P.V.I. in the last four special elections, that “Democrats may still be locked in a 2010-type political environment.”