Donald Trump often receives high approval marks from members within his own party, a sign many interpret as a forceful demonstration of strong party loyalty in the current age. Moreover, many view this strong base support as a constraint on other Republican elites; despite a tumultuous presidency, elected Republican officials must heed the opinion of their rank-and-file and cannot abandon Trump. However, many have also raised questions over the reliability of these intra-party approval numbers. Specifically, a key question–that I have tried to speak to before–is whether partisanship is endogenous to Trump approval. If original Republicans who approve of Trump continue to identify as Republican but original ones who disapprove of Trump start to eschew this label, then this creates a misleading portrait of base support. In such a case, Republican identification starts to become inseparable from support for Trump, and thus party breakdowns of Trump approval lose meaning.
Recent panel survey data from Democracy Fund’s Voter Study Group can shed light on this important question. I’ve used this data often, but crucially, this recent release includes a wave in July of 2017. It of course retains its panel structure, providing measures of partisanship in December of 2011, December of 2016, and July 2017 on the same 5,000 individuals. Below, I break down Trump approval (approve and disapprove percentages) asked in the July 2017 wave by these three measures of partisanship. This allows for an intra-party measure of Trump support that few if any current polls can manage–specifically, answering the question of how differently Republican Trump approval (during his presidency) would look like among individuals who originally identified as Republican (i.e., an earlier measure of partisanship, such as in 2011).
The key comparison is between 2017 Trump approval among “original Republicans” (individuals who identified as Republicans in 2011) and 2017 Trump approval among “current Republicans” (those a part of the party in 2017). The more favorable current Republicans’ approval is than original Republicans’ approval, the more evidence accrues in favor of the theory of endogenous partisanship inflating Trump’s base support. If approval ratings among the two groups are similar, then perhaps endogenous partisanship does not present much of a concern. Approval is indeed worse when using this original party ID measurement (79 percent) than when using contemporaneous partisanship to break down Trump approval (83 percent). These percentages are statistically significantly different at p < .01.
However, in a substantive sense, these approval levels are very similar. About four of five Republicans approve of Trump regardless of whether current or earlier measures of people’s partisanship are used. This suggests that partisanship for the most part is not that endogenous to Trump approval, and that high base support for Trump observed in current approval polls during his presidency is not inflated (e.g., HuffPost Pollster data on Republican approval of Trump currently pegs him at 83 percent approve/15 percent disapprove). Caveats–this is just one survey, the most recent Trump approval measure is from July 2017 and things may have changed since then, and possible survey attrition interfering with results–are always in order, but political observers should largely view Trump base support numbers as meaningful.