Summary. There are two take aways. First is the odds of a populist getting elected goes up as GDP growth drops. (Oddly, there is he doesn't graph GDP per capita.) Quoting:
I found limited evidence that links macroeconomic conditions to populist electoral success. The exception to this was annual GDP growth, which, unlike inflation, unemployment, corruption, GINI or trade openness, is significantly linked to the odds of a populist being elected. In particular, a 1% increase in GDP reduces the odds of a populist being elected by 12.5%, within the next zero to five years.
Secondly, the effect is non-linear. This is important for Trump. While Trump did see a drop and GDP and Biden has seen an increase, the effect wasn't huge and those first few percentages of change don't matter much. It's big changes that see a politician's popularity get drive to remarkable highs or all time lows.
As an aside the article is unnecessarily hard to read. It could really use being put through a Readability Level calculator, and getting is score down.
