No, it was not just Jeremy Corbyn. Last week’s crushing defeat of the left was also caused by the Liberal Democrat party splitting the vote. Yet again, by offering itself as the vote launderer of British liberalism, it has served as merely the fellow traveller of British Conservatism. Boris Johnson owes it a huge debt of gratitude.

Misreading election results – largely so as to gloat or spread blame – is the traditional folly of post-election analysts. Boris Johnson won the election with a thumping parliamentary victory, but the operative word is parliamentary. His 43.6% of the vote was ahead of Theresa May’s only by 1.2 percentage points, and dozens of his MPs were elected with less than 50% of the vote. Most of these – such as Kensington, Keighley, Bridgend and Chingford – were seats that Labour would have won had there been no Lib Dem presence.

As for the election being a “second referendum” on Brexit, anti-Brexit or second referendum parties won more votes than did Johnson, even assuming all Tories were pro-Brexit. Yes, leave voters appear to have swarmed to the Conservatives, notably in the north, and are thus probably short-term. But the Lib Dem vote soared by 1.3m or 4.1 percentage points, while Corbyn’s fell by 2.6m. Johnson’s rose by only 304,000. The reality is that the left-of-centre vote was calamitously split. Polling during the campaign saw Labour surge only when the Lib Dems appeared to collapse.

With the exception of the Tony Blair era, Labour has been persistently vulnerable to moderate leftwing sentiment defecting to the Liberals. Somehow such voters feel they can exonerate themselves from guilt by association with socialism without overtly siding with conservatives. They can comfort themselves that they voted “a curse on both houses”. In reality they did not. They effectively voted Conservative, or in a very few Lib Dem seats, they voted “hung parliament”.

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With the Lib Dems leaderless and in disarray, it must be time for them to do the honest thing and disband. They should reverse the old 1983 SDP marriage, and merge with a revivalist Labour. It would galvanise Labour’s moderate wing and dilute the influence of the Corbynites.

The Liberals have never looked like winning an election since Lloyd George’s day. They have offered an electoral dustbin between political polarities, a media-cosseted Westminster club with a peerage auction for richer donors on the side. Liberals failed to capitalise on regionalism or nationalism. They were never truly radical, and in coalition were a broken reed. For the past year, the party has been a convalescent home for wounded warriors from the major parties, a gesture voters have treated with derision.

This party is an anachronistic political spoiler. Its time is surely over. Liberal Democrats should disperse and confer their moderating influence on two bigger parties, who badly need it.

• Simon Jenkins is a Guardian columnist