In an NBER working paper, Gino Gancia, Giacomo Ponzetto, and Jaume Ventura propose a theory of declining public support for economic unions: Broad gains from trade in differentiated goods make way for distributive conflict due to specific factors:
… this is partly due to the growth of trade between countries that are increasingly dissimilar. … political support for international unions can grow with their breadth and depth as long as member countries are sufficiently similar. However, differences in economic size and factor endowments can trigger disagreement over the value of unions between and within countries. The model is consistent with some salient features of the process of European integration and statistical evidence from survey data.