The 2016 U.S. House election was a better election for incumbents than 2014, and one in which the nation was split down the middle. The incumbency bump added eight points to the average incumbent’s margin of victory and only 12 (3%) of seats changed hands.
Only 8 of 387 incumbents contesting the general election were defeated. That amounts to an incumbency re-election rate of 98%.
In all modern U.S. House elections, the vast majority of incumbents have been easily re-elected. For many representatives, their districts would be safe for almost any candidate from their party.
But it is not only district partisanship that provides safety for incumbents. Incumbency is an electoral advantage in itself. Incumbents typically gain that advantage through greater name recognition, campaign fundraising ability, more press coverage, "franked mail" privileges, more experienced campaign operations, and ongoing delivery of constituent services for their district. In addition, incumbents may have had opportunities to help shape the boundaries of their home district to their advantage.
We can quantify the incumbency advantage or “bump” by calculating the difference between incumbents' winning percentage and their district’s projected partisanship. In the seven U.S. House elections from 1996 to 2008, for example, incumbents on average had a winning percentage nearly seven percent higher than their district's partisanship would have predicted. This would translate into a 67% to 33% win in a district that favors their party 60% to 40%, and an average victory margin boost of 14% over a the likely result in that district if it were an open seat.
In recent years, the incumbency advantage has declined. In 2010 and 2012, the incumbency bump was less than 5% in both years. In 2014, the incumbency bump declined to less than 3%. In 2016, early results indicate the incumbency bump rose to 4%, providing an incumbent an additional 4 points of the vote over the hypothetical open seat candidate from their party.
Measuring Incumbency Bumps to Determine Partisan Swings: Given that incumbents of both parties are able to secure the same incumbency advantages in comparable ways, the differences in incumbency advantage by party give an indication of the underlying partisan preferences in a given year. In 2008, for example, the median Democratic incumbent ran 10% higher than their district's partisanship, while the median Republican incumbent only received a 2% bump, in a year with a median incumbency bump of 6%. This suggests that the underlying partisan preference of voters that year was 54% - 46% Democratic. In 2016, early data indicates that Republicans and Democrats received the same incumbency advantage, indicating the underlying partisan preference of voters was split evenly, 50% - 50%.
Support for the Parties in 1996-2016: The partisan preferences of voters in the last eleven elections favored Democrats six times, including in 2012, and Republicans five times, including in 2014 and 2016. But only three times has one party had a preference edge of more than 52% to 48%: Democrats in 2006 (53%) and 2008 (54%) and Republicans in 2010 (54%). The fact that Republicans in 2017 will have a secure hold on the House is tied to the fact that the median district is a 53% Republican district. All things being equal, therefore, Democrats need to have a particularly strong national advantage to overcome that Republican bias - as they did in 2006 and 2008.