First, non-voters are — on average — only a bit more Democratic than voters, and a bit more supportive of liberal policies. For evidence, see my work with Jack Citrin and Eric Schickler, this paper by Jan Leighley and Jonathan Nagler (now part of their book), the chapter by Eitan Hersh and Steve Ansolabehere in this book, and this paper by Ben Highton and the late Ray Wolfinger (helpfully cited by Dylan Matthews).
Now, note the fine print. “On average” means that Citrin, Schickler and I found that in some states in some election years, non-voters were actually a bit more Republican. That wasn’t the norm, but we should not assume that higher turnout or compulsory voting would always and everywhere benefit Democrats. (See also this piece by James DeNardo.)
And “a bit” means that the observed differences are not always large — usually a single-digit number of percentage points, and quite often in the low single digits. Any larger differences — as in this 2012 Pew preelection survey of self-reported likely voters and likely non-voters — are the exception, not the rule.
Here’s the second important finding: simulations suggest that compulsory voting would change the outcome of very few elections. This is not only because non-voters often aren’t that different than voters, but also because lots of elections — such as at the House and Senate levels — aren’t that close.
For example, when Citrin, Schickler, and I studied 246 Senate elections from 1990-2006, only eight outcomes changed when we simulated the impact of universal turnout. (See our chapter in this book.) In a very close presidential election, as in 2000, universal turnout could have made a difference. But such elections aren’t common.
Based on an alternative method of simulating universal turnout, Thomas Brunell and James DiNardo reach a similar conclusion. They put it thus:
Higher turnout in the form of compulsory voting would not radically change the partisan distribution of the vote.
The same is true in other countries as well.
It’s also important to separate the hypothetical of universal turnout from the hypothetical of higher, but not universal, turnout. Whether higher turnout would help Democrats or Republicans will depend a lot on which voters are mobilized. Anthony Fowler has found that “marginal voters” — the kind of voter that might be dissuaded from voting by, say, a rainy day — do lean Democratic.
And obviously mobilizing exclusively Democratic-leaning constituencies — minorities, the poor — could help Democrats. But even there it depends a lot of how many are mobilized, in what races, and so on. Citrin, Schickler and I did some simulations where we eliminated turnout disparities between blacks and whites as well as among voters of different income levels. This affected the outcome of very few Senate races.
Moreover, it’s not even clear that making it easier to vote would mobilize Democratic-leaning constituencies in the first place. Adam Berinsky argues that electoral reforms tend to exacerbate, not mitigate, the socioeconomic biases in the electorate.
There’s one big challenge here, of course: we can only simulate a hypothetical like “what would happen if everyone voted?” We don’t really know what would happen if compulsory voting was the law. Different candidates might emerge. Parties could alter their messages, strategies and tactics. As a social scientist, I would be fascinated to see how compulsory voting would play out.
Nevertheless, the lesson of the political science research is much more qualified than a headline like “If everyone voted, progressives would win.” If everyone voted, a lot would be the same.