C LOSE 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. The survey includes demographic data such as race and age, as well as participants’ recollections of whom they voted for and verified records of whether they voted. In general, voters and non-voters from similar backgrounds had similar opinions. 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.

Non-voters are relatively uneducated, young and non-white. The first of these traits predicts conservatism, but the others point to liberalism. If everyone voted, 30% of voters in the 21 most competitive states would not be white, up from the actual figure of 25%. As a result, in a typical cycle Democrats would add 50 electoral-college votes—enough to reverse the result in 2016.

If voting were universal, parties would not benefit from whipping their bases into a frenzy. Instead, they would need to court swing voters, pushing policies towards the centre. That centre, however, would sit to the left of its current position, putting the brunt of the adjustment on Republicans.

The Economist’s analysis reveals that America’s Democrats would be major beneficiaries of universal voter turnout. However, we also found a myriad of ways that either party could gain or lose votes if participation increased only among certain groups. Our approach allows us to quantify the difference between 100% and 50% turnout for working-class whites, for example. The interactive chart below presents this and similar scenarios. ◼

Sources: Co-operative Congressional Election Study; Census Bureau; YouGov; The Economist