Elliott Morris points us to this magazine article, “If everyone had voted, Hillary Clinton would probably be president,” which reports:
Close observers of America know that the rules of its democracy often favour Republicans. But the party’s biggest advantage may be one that is rarely discussed: turnout is just 60%, low for a rich country. Polls show that non-voters—both people uninterested in voting and those blocked by legal or economic hurdles—mainly belong to groups that tend to back Democrats.
What would change if America became the 22nd country to make voting mandatory? To estimate non-voters’ views, The Economist used the Co-operative Congressional Election Study (CCES), a 64,600-person poll led by Harvard University. . . .
Using a method called “multilevel regression and post-stratification”, the relationships between demography and vote choices can be used to project state-level election results—and to estimate what might have happened in the past under different rules.
And, from Morris’s writeup of the methodology:
We set out to use statistics to provide an empirical answer to the following question: what would happen in American presidential elections if voting was mandatory, as it is in some other countries?
This set off a months-long inquiry that was much more difficult than we anticipated. . . .
We soon discovered that none of the commonly used computational tools in our arsenal would do the trick. Simple summary statistics and regression analysis were obviously not enough; although we could use public polling to make predictions for individual citizens, the country doesn’t elect its president by popular vote. Even popular machine-learning algorithms were not sufficiently suited for the task. . . .
Ultimately, what we needed was a technique to make predictions for each state under varying degrees of voter turnout, to figure out the electoral-college winner under a system of mandatory voting. The method would have to account for many factors, such as increasing turnout among minorities, who vote less often but lean to the left, and higher turnout among whites without degrees, who lean to the right. We also had to answer the crucial question of whether a voter and a non-voter with the same demographic profile would vote in similar ways (for the most part, they do). More questions popped up along the way.
OK, at this point I think you can anticipate what’s coming, but I’ll tell you anyway:
A solution was lurking in the background, but The Economist had never attempted it before: a statistical method, popular among leading quantitative social scientists, called “multi-level regression and post-stratification” (MRP, or “Mr P” among its super-fans). It involves combining national polls with information about individual voters to make predictions at different geographic levels. . . .
With the CCES alone, we could assess the relationship between demographics, turnout and vote choice. But due to small sample sizes in select states . . . we could not make reliable state-level projections. . . .
He even includes rstanarm code!
Regarding the substantive question, the last time I looked at what might happen if everyone voted was in 2007. As I recall, a key conclusion was that higher turnout would not just change the Democrat/Republican split, it would also change what issues would get discussed in politics.
Regarding your grid of maps: I like this sort of thing, and I like that you used a bi-directional color scheme of the sort that we have used. My only recommendation is for you to put the labels on the top and left of the grid rather than putting a label on each map. See the grid of maps from this 2010 paper for an example. Our grid from that 2010 paper is not perfect—in particular, our state borders are distractingly dark—but at least you can get the point regarding the labeling of rows and columns.
I also liked your comment, “this is not an endeavour for Ockhamites; there is danger in being too simplistic.” Here’s something I wrote about this awhile ago. Using a simplified model can be a sensible practical choice, and it should be understood as such.
That Highton and Wolfinger article from 1999 [see link and discussion in 2007“>here] was key to our approach. If we remove the assumption that non-voters behavior like voters once we control for demographic and political variables, the whole thing falls apart. With our time constraints, we also found it impossible to suss out the downstream effects of universal turnout on things like party messaging and the median voter, but we took that a shift more toward Democrats implies a shift to the left. And more toward your point, this would come with increased salience in issues that matter to poor and non-white Americans, too, I think. We did game out how Republicans might go about achieving electoral success again and found that increasing their margins among non-college whites is the most bang-for-their-buck, given the size of the group (about half of all voters, by our math).
Also relevant: The Electoral College magnifies the power of white voters.