Clarifying the Relationship between Partisanship and 2016 Vote Choice with Panel Data

When looking at polling crosstabs with party identification breakdowns of key variables like vote choice and approval ratings, many often conclude a strong impact of partisanship. This data shows that Democrats largely express support for Democratic candidates in elections and leaders, while Republicans express support for their candidates and leaders. Yet conclusions about this relationship could suffer from the issue of endogeneity (I’ve discussed this before here). Inferences about how partisanship affects an outcome Y need to assume stable partisanship, and that the outcome Y–such as vote choice–does not in turn affect partisanship. The possibility of reverse causation would mean that people first arrive at a decision to vote for Donald Trump, for example, and subsequently update their partisanship to match with their candidate preference. If this occurs, then the previously assumed “exogenous” nature of partisanship as an unmoved predictor becomes dubious. Maybe individuals who were originally Republicans but did not support Trump changed their partisanship, and as a result, partisanship became a mere reflection of vote choice, and not a stable underlying predisposition in a way that would be meaningful.

A similar concern has been raised regarding approval rating polls showing strong intra-party support for Trump. Original party base members may no longer identify as Republicans on surveys, and thus Republican party identification simply comes to mean support for Trump and not a meaningful underlying political trait. Cross-sectional survey data cannot overcome this problem, as it lacks a measure of an individual’s preexisting partisanship. Panel data, on the other hand, can better address this issue. Cross-sectional data uses contemporaneous 2016 measures of partisanship and vote choice (recorded at the same time) to say that 90 percent of Republicans voted for Trump. But the better approach would be to use a pre-2016 measure of partisanship–unaffected by Trump–and calculate how vote choice breaks down along this variable that better represents an underlying indicator of partisanship. Publicly available panel data–with waves in December 2011, November 2012, and December 2016–from the Voter Study Group (VSG) offers such an improvement in capturing party voting (the rate at which partisans vote for co-party candidates).

Specifically, I can compare how party voting looks like using both a 2011 measure of partisanship (before both the 2012 and 2016 elections) and a 2016 measure of partisanship (that is purportedly endogenous to 2016 vote choice). If there are large differences in party voting across these measures, then an endogeneity problem exists, suggesting that partisanship is shaped in response to vote preference. If small or no differences result, then partisanship constitutes a more exogenous variable–in line with the stable over time character that much of political science literature (and other evidence) suggests.

The VSG data offers mixed evidence but mostly lies in favor of the latter conclusion. I’ll start with the broadest perspective–overall rates of party voting: Republicans voting for Trump and Democrats voting Clinton as a percentage of all partisans who reported voting in 2016. If I use the 2016 measure of partisanship, I find that 89 percent of partisans voted their party in 2016. If I use the 2011 measure of partisanship, 84 percent of partisans voted their party in 2016. Thus, it appears that a small percentage of people shifted their partisanship to match their vote preference in 2016, and in a way that would slightly inflate the impact of partisanship on voting. But it’s fairly small, as the party voting rates remain similar.

Breaking these party voting rates by party and candidate reveals a similar picture, but with some additional information. The 2016 measures of partisanship suggest very high rates of party voting, with 90 percent of Democrats voting Clinton and 88 percent of Republicans voting Trump. That rate declines a bit when using a pre-Trump (2011) measurement of partisanship: 83 percent of original Democrats opted for Clinton, while 84 percent of original Republicans went Trump. These percentages are not that different, but at least some partisanship updating is likely at play. What’s more interesting is how this pre-Trump underlying partisanship better captures defection from the Democratic Party (in a way that–not balanced out by similar defection among Republicans to Clinton–could have tilted the election just enough to Trump). If we use a 2016 party measure, then we would conclude that seven percent of Democrats voted Trump. Using the 2011 measure of the original Democratic party base, however, nearly doubles that size, revealing 13 percent of Democrats who voted for Trump in 2016. This panel approach can thus offer a more meaningful estimate of how many original Democrats defected from the party in voting for the out-party candidate in 2016.

Finally, I wanted to further break down this comparison by using the full seven-point party identification scale. The below plot shows how each party identification group (of the seven in total) voted in 2016 when using an individual’s 2016 reported partisanship.


Differences in party voting rates at this partisanship subgroup level appear when comparing the above plot to the same breakdown but with an individual’s 2011 reported partisanship, as the below plot illustrates:


If we use 2016 cross-sectional data, then we end up with an overestimate of how closely “Strong Democrats” and “Not very strong Democrats” adhered to their party affiliation for deciding whom to vote for. This approach would say that 97 percent of strong Democrats and 79 percent of weak Democrats voted their party in 2016, as opposed to 90 and 70 percent respective rates when using underlying (2011) partisanship. Similar differences appear on the Republican side, but as mentioned earlier, the magnitude is smaller. The biggest difference is for the “Lean Republican category,” as while a 2016 measure suggest 90 percent of this group went Trump, a metric capturing original members of this category suggests 83 percent did.

In sum, this comparison does suggest some partisanship updating to accord with vote choice took place, but not to any large extent. Concerns with endogeneity should be tempered. That’s in large part because partisanship remains a very stable over time variable–at both the aggregate and individual level. To underscore this latter point, I used all three survey waves of the VSG (bringing in the 2012 wave that I’ve exclude up until now) to track individual level partisanship dynamics at three different points in time over a five-year span. That results in the following table, which shows the distribution of VSG survey respondents by the different possible party ID combinations they can have across the three survey waves. In each wave, they can express three different partisan affiliations, which makes for 27 unique combinations (3*3*3 = 27).


Two combination groups stand out: people who identified as Democrats in all three years (41.74 percent of all respondents) and people who identified as Republicans in all three years (33.79 percent). That means about three out of every four people (75.53 percent to be exact) are consistent partisans over the course of four years. The next most common group is people who do not reveal any partisan leanings–Independents–which makes up 6.07 percent of all respondents. Thus,  81.60 percent of all people have a consistent expression of partisanship (or lack thereof) across five years at three different points in time, a piece of evidence indicative of strongly stable partisanship.

Moreover, looking further down the table, only 4.64 percent of respondents ever identify with both parties at some point during the three survey waves. Rather, most of the party switching–which is very little to begin with–is into and out of the Independent category (this movement takes up 13.76 percent of all survey respondents). In light of these trends, the lack of substantial endogeneity–changes in partisanship driven by vote choice selection–should not come as much of a surprise.

Clarifying the Relationship between Partisanship and 2016 Vote Choice with Panel Data

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