For any modern or contemporary historian, the United Kingdom Independence Party (Ukip) irresistibly brings to mind the Social Democrats of the early- to mid-1980s (above). It's not the Guardian reading or the moderation, of course - not the calm teacherly voices and the long deliberation of that most democratic and centrist of breakaway parties - that suggests the parallels.
It's the electoral dynamics of breaking away from one part of the electoral landscape, and trying to fill both that gap and the rest of the vessel of democracy at one and the same time. The SDP broke away from Labour, and nearly killed its 'parent' party in the act. Ukip has its origins in right-wing revolts: on immigration, on Europe, on gay marriage, and on a rather vague and nebulous sense that 'the elites' are betraying 'the people' (whatever that means). But it knows that it must invade Labour's blue-collar fortresses as well if it's to have any lasting future.
What it most needs to do this is what someone once called 'the oxygen of publicity'. Its vote share shoots up when it does well in local or Euro elections, but then trends down again when it's out of the headlines. Hence the recent defections of Eurosceptic Conservative MPs such as Douglas Carswell and Mark Reckless, and the by-elections we're now going to be treated to in their seats (Clacton and Rochester and Stroud). And that was exactly what happened to the early SDP. It shot into the headlines winning Glasgow Hillhead and Bermondsey in spectacular by-election coups at either end of the country; it won both Conservative-held and Labour-dominated seats; it seemed to appeal to everyone who was sick and tired of the two older and (by that time) apparently extremist parties.
But the SDP ultimately spluttered and died. For a moment in early 1982, it really did seem as if the SDP and their Liberal allies might win the forthcoming General Election - or at least be in a position to form a coalition with Labour. Mrs Thatcher's resurgence, given the economy's recovery and the Falklands War, put paid to all that. But losing by-elections also did it. Mitcham and Morden in June 1982 and Darlington in March 1983 (that latter contest often credited with allowing Labour leader Michael Foot to keep his job) took lots of the steam out of their engine when the Conservatives and then Labour triumphed. The same could happen to Ukip if they fail to hang on to Rochester and Strood. Clacton looks gone, right at the top of academics' '100 most likely Conservative seats to go to Ukip'. But Rochester and Strood, which sits precisely nowhere in that list, is less likely and fertile ground. Lose it, and all the air will go out of the balloon. Win it, and other defectors and other by-elections are likely completely to derail the Conservative electoral push. The stakes are high.
Ukip actually has a lot going for it. Two decades of machine electioneering, perfected especially by the Liberal Democrats, which show that you can win seats if you target all your resources on them - rather than hoping for the SDP's larger, and in the end chimerical, 'breakthrough' everywhere. The new party has a heartland - roughly, poorer eastern seaside and inland towns with high proportions of low-income voters. They know where they can win: Clacton and Rochester and Strood, of course, but also in Thurrock, Great Grimsby and Thanet South (where Ukip's leader, Nigel Farge, is standing). Some unlikely fruit might just fall into their laps - Lord Ashcroft's recent polling of Liberal Democrat seats suggests that they're in with a shout in Torbay and St Austell and Newquay in the South West (you can download the full report here). They'll probably win two to four seats if they go on like this. But it's like threading the proverbial eye of the needle - one big misstep, and the whole prospect might disappear.
So what must they do? Win Rochester and Strood, and secure one or two more defections. Keep up the momentum, in terms of plausible narrative at least. If they once look down, or look like they are wobbling in the performance of their high-wire act, the precedent of the SDP suggests that they might fall - far and fast.
The Conservatives had better hope they do. If they don't? Ed Miliband will walk into Downing Street, almost unopposed.
