Presidential approval matters in midterms -- but how?

Political scientists and analysts have puzzled over this question for years in academic journals and other wonkish publications. In July, I made a small contribution to the large body of existing thought, essentially finding that in recent, competitive midterms where no major events disrupted presidential approval (e.g. 9/11, a Monica Lewinsky scandal) the incumbent party typically had trouble surpassing the president’s approval rating in generic ballot polls.

So it’s worth asking -- is the same thing happening to the GOP and Trump? Is his approval rating (which is, at the moment, low) a ceiling for House Republicans?

Obviously it’s too early to answer this question definitively. The midterms are over a year away, and events could change any of the numbers we’re examining here. But it’s worth periodically checking in on this theory and seeing if it holds.

That’s why I put together this graphic:

It shows the gap between Trump’s approval rating and the Republican’s share in each generic ballot poll taken between Inauguration Day and Aug. 20 (seven months later).

The key takeaway pops out quickly -- almost all of the points are greater than zero. That means Trump’s approval has been equal to or greater than the GOP’s share in generic ballot polls. In other words, the first few months of the Trump presidency have basically cohered with the theory that the president creates the in-party’s ceiling.

It’s also worth noting that the gap isn’t large. Typically Trump only outperforms the GOP by a few points. And if we use other metrics (e.g. compare the daily RCP tracking polls for the last three months) that difference can disappear. So if there’s a real gap between Trump’s performance and the GOP’s, it’s small.

That’s not to say that this pattern will hold on Election Day. I performed a similar exercise with the Democratic share of the generic ballots and presidential disapproval -- and presidential disapproval far outpaced Democratic numbers. It’s possible to imagine a scenario where some voters who dislike Trump end up voting for Republicans because they dislike House Democrats even more. It’s also possible that some not-yet-understood aspect of the Trump presidency will cause a break from the historical pattern.

But that’s exactly why I want to check on these numbers periodically -- to keep track of which rules Trump can break and which ones he can’t. If the 2018 election turns out to be a referendum on him (a close relationship between Trump’s approval and the GOP share of the generic ballot vote would support this theory), then we’ll know that historical patterns held and, with respect to midterms, Trump is a regular politician. And if this relationship doesn’t hold, we’ll be able to learn something new about a president who breaks some political “laws of nature” but has been bruised in his attempts to break others.