IT WAS IN the 1970s that American politics began to polarise around voters’ levels of educational attainment. The Republican Party, until then a party of tweedy north-easterners, began recruiting less-educated southern whites, alienated by the civil-rights movement. Over time, the partisan gap between college-educated voters and less-educated ones widened. In 2016 it exploded. The Pew Research Centre, a think-tank, found that overall, college graduates favoured Hillary Clinton by 21 percentage points, while those without a degree backed Donald Trump by a seven-point margin. Among whites, the difference is greater: those without a college degree backed Mr Trump over Mrs Clinton by a margin of more than two to one.

How far did this educational divide determine the outcome of the 2016 election? To answer this question, Michael Sances of Temple University collected data on presidential-election results and education levels in each of America’s 3,000-plus counties from 1972 to 2016. Mr Sances finds that the gap in support for Democratic candidates between the highest- and the lowest-educated counties grew significantly between 2012 and 2016, from about 16 percentage points to 28 percentage points (see chart). This disparity has grown especially quickly in midwestern swing states. In Iowa, for example, Hillary Clinton won 66% of the vote in better-educated counties, up from Barack Obama’s 61% share in 2012, but only 27% in less-educated ones, down from 46%.

According to Mr Sances, this shift in political support was decisive to Mr Trump’s victory in 2016. Had the counties in the bottom 10% of the education distribution stuck to their voting behaviour in 2012, Mrs Clinton would have been tied with Mr Trump in the electoral college. Had the counties in the bottom 20% done so, she would have won. As the 2020 election approaches, Democrats will have to think seriously about how to bring less-educated voters back to their side. The party’s current focus on left-leaning government programmes such as Medicare for All, which tend to be popular with well-educated liberals but poll poorly among blue-collar white voters, are unlikely to tilt the scales back in their favour.