For instance, in Rotherham last week, the Lib Dems got 2.1% of the vote on a 33.9% turnout – that’s 0.7% of the electorate. Back at the 2010 general election, they had managed 16% of the vote on a 59% turnout – that’s 9.4% of the electorate.
So they lost 92% of their previous support. On the same measure, the Conservatives dropped 81% and Labour – even though their vote share was up – dropped 53%.
The other defining feature of byelections is that they’re local elections for local people, and you shouldn’t generalise from any one result. But averaging across all the byelections that happen during a parliament is a bit more informative.
The chart below shows how much of their previous vote the parties lost (or, in some cases, how much they added to it) at byelections in the last eight parliaments:
The obvious points are that governing parties do particularly badly, and the Lib Dems (or their predecessor parties) have sometimes made impressive gains. Until now.
After only two years in government, the Conservatives are doing as badly as John Major did, and the Lib Dems are doing even worse. Labour are doing better than when they were in power, but not as well as during most of their last period of opposition.
Update: I've been asked if I could do a similar chart showing the conventional change in the share of the vote - so the Lib Dem fall in Rotherham from 16% to 2% would count as -14%. I think this is less significant, because the numbers are more easily distorted by the variability of the starting points of the constituencies that happen to have byelections.
For instance, across the 12 seats that have had byelections since 2010, the average starting point was Lab 46% Con 24% LD 19% - an unrepresentatively low start for the Tories, meaning that in these seats, there was a limit to how far they could fall. That's why I looked at how far the parties had fallen relative to their original positions.
But with that caveat, here's the chart:
And, while I'm here, below is a version of the first chart but disregarding changes in turnout. It shows the change in vote share as a percentage of the previous general election vote share. So the Lib Dem fall in Rotherham from 16% to 2% of the vote counts as -87%.