The combined Labour-Conservative vote share dropped to just 63% in today's YouGov poll, the lowest in this parliament. Lord Ashcroft even had them on 60% this week. Here are some charts to put this decline of the ‘big two’ in a historical perspective. First of all, a 10-poll rolling average shows that it is mostly the fall in the Labour vote share that has driven the most recent part of decline, over the last 12 months. The Tory vote share has been more stable:
This has actually been the case for more than two years, since the ‘Omnishambles’ budget. The long-term chart shows a slow decline since 2010, but not yet to the same extent as during Cleggmania, when a number of pollsters had the combined LAB-CON combination down in the 50s (YouGov had a low of 57):
It also shows that these types of dips have happened before. European election periods seem particularly conducive to them, as smaller parties get increased attention. But a more prolonged dip occurred in the mid 1980s, when the Alliance was particularly strong. But of course, none of this approaches the situation at the height of the Alliance surge in December 1981 (after the Crosby by-election) when Gallup had the Liberal-SDP combined share at 50.5%.