The Hartlepool by-election result is a landmark in British politics. Not only is it the first time the Tories have won the seat since its creation in 1974, it is also the largest percentage point increase in the vote share for any UK governing party in a postwar Westminster by-election.
Opposition parties tend to outperform governing parties in by-elections, having won around four in every five by-elections in the postwar period. Even when incumbent parties have won by-elections in the past, it has never been on the scale of the Conservatives’ Hartlepool victory.
The largest previous surge in vote share was for Labour in the 1945 Bournemouth by-election. But even then, the increase was around half the 23 percentage point gain in Hartlepool, and the Labour candidate still lost to the incumbent Conservative MP.