The Liberal Democrats are back at the seaside for their first annual conference after two seismic events. The party was at the epicentre of one – May’s general election, when its parliamentary base was devastated. As for the other – the extraordinary elevation of Jeremy Corbyn as Labour leader – the Lib Dems are interested observers.

The combination of shock and aftershock define the challenge facing Tim Farron, named over the summer as successor to Nick Clegg. There is no silver lining around the loss of 49 out of 57 MPs, coming after years of attrition in local government. The task here is slow reconstruction from the ground up, combined with the job, no easier, of defining what purpose the Lib Dems usefully serve. But in the case of Mr Corbyn, there is an opportunity, first advertised with premature optimism by Mr Farron in Friday’s Guardian.

This opening lies in the alienation of some former Labour supporters and swing voters who will be baffled and perhaps appalled by the direction the party has taken. Many will be hostile to the current government, either because they hate its policies or because of a deeper cultural inoculation against ever voting Tory. It is feasible to imagine the Lib Dems as a repository for those unallocated votes; the question is what platform achieves that goal. The Lib Dems’ strongest election results were notched up under Charles Kennedy thanks to former Labour supporters who could not stomach Tony Blair’s foreign policy, disdain for civil liberties and acquiescence in the Thatcherite economic legacy. That was a flight by Labour’s left, and many such people will feel comfortably rehoused under Mr Corbyn’s roof.

Meanwhile, Mr Farron’s base is on the Lib Dem left. He was chosen partly in repudiation of the compromises Mr Clegg made with Conservatism. He defended the record in government but his style owes more to the campaign-and-complain mode of pre-coalition days. His stated intention is to take a handful of positions – support for immigration, housebuilding, civil liberties, EU membership – and make them emblems of principle inadequately protected in the bigger parties. But as a serial protester, or conduit for righteous indignation, Mr Farron will be easily outflanked by Mr Corbyn.

The vacancy is for a party that marries Mr Farron’s campaign themes to a practical account of how to achieve them, with more respect than Mr Corbyn seems to show for the sensibilities of moderate opinion and the practical policy constraints, including finite budgets, and a recognition that the state needs modernisation as well as investment. Tonally, it must eschew the nostalgic, tribal certainties that are found on both left and right. Since Mr Corbyn dislikes markets and capitalism, the Lib Dems have room to argue, as New Labour once did, that they can be harnessed to progressive goals. Awkwardly, this “third way” looks much like Mr Clegg’s message in an election that ended in calamity – more compassionate than the Tories; more prudent than Labour. Trying to be all things to all people, ending up not pleasing anyone. Mr Farron, free of Mr Clegg’s coalition baggage, must rehabilitate that strategic position but as an expression of free-standing liberalism, defined on its own terms, not simply calibrated to compensate for other parties’ weaknesses. It will not be easy but, since the alternative is obsolescence, it is the path Mr Farron must walk.