It is just over 20 years since David Owen's Social Democratic Party came seventh, behind the Monster Raving Loony Party, in a by-election in a rock-solid Labour seat in the north of England. Dr Owen got the message, and a few days later, the SDP ceased to be.
An almost identical disaster has now befallen the Liberal Democrats, but the effect will not be so terminal. Their very poor showing in the Barnsley Central by-election does not signal the death of Liberal Democrat England, nor a return to two-party politics. But it is a startling humiliation. There cannot be anyone who seriously expected their candidate to trail in sixth place with a lost deposit, beaten by the UK Independence Party, the BNP and an independent candidate.
This is a very bad portent indeed for hundreds of Liberal Democrat councillors who have to defend their seats in local elections on 5 May, and a hard lesson for a party which, as constituted at present, had never been in government before last year. At a time when professional politicians generally are unpopular, the Liberal Democrats can no longer pose as the clean alternative to dirty party politics. They are part of the system, the junior half of a government making difficult and unpopular choices. They are not alone in seeing their support ebb away – the Conservative candidate in Barnsley was beaten into third place by Ukip – but they have taken the bigger hit because the policies they have endorsed in office are so different from what they said in opposition.
However, Nick Clegg was right to warn people not to "write off" the Liberal Democrats. It is in the nature of democratic politics that governments cram their least popular decisions into the early years of a new parliament in the hope that they pay in the form of rising popularity when the next general election is imminent. The Liberal Democrats are taking more than their share of the blame; they can reasonably expect their share of the credit when things are looking up.