However Ukip fares in this week's elections, the politics of protest can only take you so far

Network Rail maintenance engineer Frank McKenna isn’t a fruitcake. He’s convincing when he says he has no time for the racist BNP. He was born in County Antrim – his father was a republican – and he lived for 28 years in Australia where he was an activist in the socially liberal third party Democrats. And apart from his apocalyptic view of the threat to member state sovereignty posed by the EU’s Lisbon Treaty, and the – to him – infuriating shenanigans which led the Irish public to reverse its original 2008 No vote and which brought him into Ukip, it would be hard to guess at first sight which party he supported.

His candidacy for a seat on Kent County Council illustrates one of the reasons for Ukip’s reaching the point where it can come second in the Eastleigh by-election and Professor John Curtice, of Strathclyde University, can say it “now threatens to pose the most serious independent fourth party incursion in English electoral politics in the post-war period”. Unlike its toxic predecessors to the right of the mainstream parties – including the BNP – the respectability of candidates like McKenna has ensured that it has not been socially embarrassing to declare, let alone vote, for it.

Until now. Nigel Farage’s defence against the increasing evidence that in parts of the country his party has been less than fastidious about the credentials of its candidates – including a few homophobes and not-so-closet racists – has been unconvincing. He complains of “a smear campaign” as if trawling through the social media utterances of candidates for public office was somehow illegitimate. And he says that the party does not have the “resources” to vet all 1,700 candidates for Thursday’s elections. Well, one answer might be not to run so many candidates until you know who they are.

Alarmed though some Ukip activists may be at Ken Clarke’s robust weekend attack, they will nevertheless comfort themselves with Farage’s mantra that it’s when you’re succeeding that you get denounced. And certainly Ukip is likely to be the story on Thursday. For all Labour’s outward relaxation, some of its senior figures are increasingly nervous that it too could lose white working-class votes to the party. But the main argument that Ukip has triggered is with the Conservatives. So what is the nature of the threat? Has Ukip merely replaced the LibDems, locked in as they are to the Coalition – as the main repository of protest votes by disaffected Tories? Or has economic slump caused the unprecedented electoral volatility visible elsewhere in Europe?

This isn’t largely about policy. Farage attacks David Cameron as “social democrat”, which contrives – baselessly – to insult both the Prime Minister and social democrats. And he has an easy answer to the “29 million” Bulgarians and Romanians who the party’s leaflets luridly warn will be entitled to come to the UK from 2014, which is to get out of the EU. But its only thanks to an EU referendum pledge by Cameron that Ukip has any chance of bringing this about. And Farage’s effort to create a national platform beyond this is, to put it politely, a work in progress.

Certainly a sniff at the opinion of those considering voting for Mr McKenna in Folkestone on Sunday morning reinforces Lord Ashcroft’s observation, from his own polling in Eastleigh, that rather than being driven by Ukip “principle”, more than four in five of Ukip voters “were protesting about something”. Indeed, some seemed to be moving towards Ukip despite, rather than because, of its policies. Malcolm Arnold, 58, much more of an admirer of Margaret Thatcher than of David Cameron, was worried about Europe but didn’t want to pull out altogether. He admitted he might vote Ukip this week as a “protest”, but would be much less likely to do so in 2015 “because it would let Labour in”. Pat, 72, would definitely vote Ukip because as a local Samaritan she didn’t like what welfare changes – however necessary – were doing to “low income groups.” She didn’t seem to have considered voting Labour, and didn’t know how she would vote in a general election.

Clarke is right that Ukip is much clearer about what it is against – whether it’s HS2 or wind farms or, in Kent, the extension of Lydd airport – than what it is for. And the argument among Conservatives about how to deal with Ukip may say most about their own party. Insofar as Ukip represents a breakaway from Conservatism, Cameron has been less able to prevent it than Thatcher (while in office) was. But those Tory MPs who argue that the answer is not to attack Ukip, as Clarke has, but to pursue more right-wing policies – especially on Europe – are sometimes using the fourth party as leverage to advance their own ideological goals, helped, of course, by those MPs who genuinely fear losing their seats to Ukip.

Those fears aren’t crazy. Curtice points out that even if Ukip’s vote share recedes to, say, “only” 5 per cent after its now likely triumph in next year’s European elections, that could still cost Cameron some 10 parliamentary seats – which he could do without given the already uphill struggle the party faces as a result of the postponement of boundary changes. That said, however well Ukip does on Thursday, it will have its work cut out to become, as Farage wants, a “mainstream” party rather than an inchoate, if potent, vehicle of protest and disaffection.