While I await some help from political science friends in evaluating Harry Enten’s argument at FiveThirtyEight that varying presidential and midterm electorates aren’t that big a deal in explaining recent election results (there’s a rebuttal at TNR today, but it’s based on the author’s access to data the rest of us can’t see), let’s look at another piece from the site which I am happy to endorse: Nate Silver’s evaluation of loose claims that parties holding the White House for two terms are cruising for a bruising.
Nate’s target in this case is Megan McArdle, who asserted that Republicans have a 75% chance of winning the White House in 2016 because people get tired of the party in power, making presidential elections “a metronome.” Is that really true? Here’s Silver’s answer:
What has happened, historically, after the same party has controlled the White House for exactly eight consecutive years?
Since the Republican Party first nominated a presidential candidate in the election of 1856, ushering in the modern two-party system, this circumstance has occurred either 11 or 12 times. The ambiguous case is in 1868. Abraham Lincoln, the great Republican president, had been assassinated and was succeeded by Andrew Johnson, his vice president, who is variously defined as a Democrat; as a member of the National Union Party, which Lincoln temporarily established in 1864; or as having no party affiliation.
Excluding 1868, the incumbent party has won the popular vote in 5 of these 11 cases, and the Electoral College in 4 of the 11 cases. Including 1868, when Ulysses S. Grant won a third consecutive election for Republicans but directly succeeded Johnson rather than Lincoln, the incumbent party’s track record improves to 6-of-12 in the popular vote and 5-of-12 in the Electoral College.
Depending on your definition, then, the incumbent party’s success rate in these elections has been anywhere from 36 percent (4 out of 11) to 50 percent (6 out of 12). Perhaps you could argue that Republicans’ odds of winning in 2016 are slightly better than 50 percent on this basis, but claiming that their odds are as good as 75 percent, as Ms. McArdle does, doesn’t seem to have any justification in this evidence.
Noting that all presidential elections aren’t actually equal, and that margin of victory is a factor in evaluating the odds of winning or losing after eight years in power, Nate runs through those numbers and concludes that with all other things being equal the odds are roughly even–in other words, it’s not demonstrably an independent factor at all.
The bigger lesson here is that it’s possible to build all sorts of dubious cases for predictions–particularly for presidential elections, which occur only every four years–if you limit the sample of elections and at the same time ignore the details. Republicans may or may not win the presidency in 2016, but they actually don’t have some sort of historic wind at their backs.