Policy or Partisanship in the United Kingdom? Quasi-Experimental Evidence from Brexit

Are voters motivated by policy preferences or partisan identities? In this paper, we argue that the British Conservative Party's sudden change in Brexit policy (following the surprising result of the 2016 referendum on EU membership) offers a unique opportunity to study partisanship in the context of a natural experiment. Using an interrupted time series design, we find evidence that voters care primarily about policy: Europhilic Conservatives disaffiliated from the party, while Euroskeptics became more likely to identify with the Conservatives. These findings suggest that voters are sufficiently policy motivated to change parties if they disagree with their party on important issues. But we find that partisan identities do play a role in the development of voter preferences in another issue area: voters who joined the Conservatives immediately after the referendum subsequently adopted more right-wing views on economic redistribution.