By Nathaniel Rakich of FiveThirtyEight
For decades, running as an incumbent was undoubtedly a huge advantage in electoral politics. As recently as 20 years ago, holding office added an average of 8 percentage points to a candidate’s margin.
But in this century, experts say, the incumbency advantage has significantly diminished. Now the verdict is in for the 2018 election: According to our method of calculating it (which is different from other researchers’, so keep in mind that these numbers can’t be compared directly to those from previous years), the electoral benefit of already being a member of Congress this year was down to less than 3 points.