Whether the Lib Dems and Labour like it or not, the best way to defeat the Tories is to join forces

In his speech to the Liberal Democrats’ conference on Sunday, Ed Davey argued: “These Conservatives can’t be defeated next time unless we Lib Dems win Tory seats. Boris Johnson will stay in Downing Street unless we throw him out.”

Although party leaders often make exaggerated claims to rally their troops, on this occasion Sir Ed’s ambitious goal was based on solid ground. In June, his party overturned a 16,000 Tory majority in the Chesham and Amersham by-election with a 25 per cent swing, prompting a panic among Tory MPs and activists in their “blue wall” in the south of England. Michael Gove, the new housing secretary, has paused controversial plans to reform planning rules to ensure more housebuilding, which the Lib Dems attacked as a “developers’ charter”.

As the Lib Dems have learnt, to their cost, it is not easy to replicate a stunning by-election success at a general election. The party has only 12 MPs and is no longer the third party at Westminster due to the Scottish National Party’s advance. The Lib Dems averaged 10 per cent in the 17 opinion polls taken last month – up slightly from 7 per cent in January but still trailing the two biggest parties. Yet Chesham and Amersham shows they are back in business and capable of playing a significant role at the next general election.