Landing on doormats across Somerset in recent weeks has been a Tory election leaflet the like of which local people have never seen before: "A vote for Ukip is a vote for the Lib Dems. Ukip has no plans or policies for Somerset. Only the Conservatives can deliver an In/Out referendum by 2017."

The wide fields and market towns of this part of the country have changed political masters six times since the 1970s, with the Conservatives and Liberal Democrats sharing the county council spoils.

Labour has traditionally struggled to find a foothold in the levels, but Nigel Farage's UK Independence party has never even won a council seat, attracted more than a few hundred votes in any electoral ward, nor looked particularly likely to make any such a breakthrough.

Yet the Tory attack dogs behind the election flier believe things are changing here, and elsewhere, and they are not waiting for Ukip to prove them right. Something is bubbling under the surface and the ragtag platoon of Ukip activists in Somerset say they feel it too.

"I have said many, many times I wouldn't count any chickens", says Dorothy Baker, the 77-year-old retired teacher and grandmother-of-six who is part election generalissimo and part self-confessed mother hen to the "'Kippers" of Somerset. "But," Baker says, patting her tummy, "down here something is saying 'Goodness'.

"It is quite humbling really. The message is finally getting across that what we are saying is true. I believe this country has been given away to a foreign power and I believe we should be fighting to get it back. That's Ukip's message."

The party, founded in 1993 in opposition to the Maastricht treaty, will have candidates in 80% of the seats in Somerset's local election on Thursday, compared with just 10% in 2009. Nationally, it will field 1,734 candidates – more than three times the 560 candidates who stood three years ago, and more than the Liberal Democrats are putting up this year.

The rapid scaling up of the party has led Professor John Curtice, a leading political scientist, to claim that Ukip presents the "most serious fourth party incursion" into English electoral politics since the second world war. Certainly, recent statistics have sent a shiver down the collective spine of strategists at Conservative Campaign Headquarters.

In November last year, in a parliamentary by-election in Corby, Ukip attracted 14% of the vote, in what was its best ever result. Two weeks later it achieved 22% in Rotherham. In March, Ukip pushed the Conservatives into third place in the Eastleigh by-election after attracting 28% of the vote, with many supporters claiming they could have gone one better and beaten the Liberal Democrats to attain their first Commons seat if they had been more organised in getting their postal vote out.

It is a remarkable turnaround for a party that has bumbled along for 20 years, often appearing to be on the verge of disbanding and at times, notably under the leadership of the TV host Robert Kilroy-Silk, verging on the ridiculous. So who is voting for them – and why?

The attraction, focus groups tell the main parties, is not just that Ukip is anti-EU, but that it isn't the Tories, Labour or the Liberal Democrats. A significant number of its voters, according to an analysis by Dr Robert Ford, from the University of Manchester, are former Tories. In 2009 roughly two in 10 Ukip voters had voted Conservative in the previous election. Now it is four in 10. But that still means less than half of Ukip's support is from former Conservative supporters. A further quarter has come from Labour and Liberal Democrats, according to Ford.

Anger over immigration, anxiety about national identity, hostility to the EU and a deep disaffection with "politics as usual", themes that cut across traditional party dividing lines, have enabled the party to recruit voters outside the small pool of white, middle-class reactionaries who have traditionally found a home with the party. A lot of them now look like Labour voters, says Ford. Farage's simple message to this burgeoning and angry group was caught succinctly by the title of his recent flesh-pressing UK "Commonsense tour". He is on their side, he insists, playing down the libertarian, anti-Brussels, anti-regulation string to his party when it suits him.

But it is in this broad but loose alliance that there is danger to the Ukip brand.

After a lunch in Westminster with a packed room of lobby journalists last week, Farage was keen to have a few drinks and was full of bonhomie, only to be shunned by abstemious hacks. He decided to catch the 5pm Eurostar back to Brussels. The hard-drinking, twice-married former stockbroker is an affable figure whom many voters warm to, even if journalists can't find time for a drink with him on a busy news day.

But the few other Ukip characters who find themselves in the public eye tend to be those plucked out by the media and the party's political opponents for their colourful and often distasteful views. Last week, in the latest of a long line of similar embarrassing revelations, Anne-Marie Crampton, a local election candidate in East Sussex, was suspended after reportedly posting antisemitic comments on a conspiracy theory website, including the claim that the second world war was "engineered by a Zionist". She claims her computer was hacked into.

Back in Somerset, the Twitter account of the vice-chairman of Yeovil Ukip, Godfrey Davey, a candidate in Thursday's election, doesn't bear much scrutiny. "At the rate this government is going we will end up with civil war it will be us or the imegrants [sic]", he writes. "Every time you give sodomites an inch they want a mile,no pun,pedeophilea here we come [sic]", he adds. He tells a Twitter account promoting Rosetta Stone linguistics software, "If you speak English,YOU DO NOT NEED ANY OTHER LANGUAGE."

It is reported that a number of the Ukip candidates for this week's election are secretly former BNP activists. Yet the Tories and the rest have learned that, the cranks apart, Ukip supporters cannot be written off any longer, as the prime minister once famously did, as a bunch of "fruitcakes, loonies and closet racists". Ukip will attract some unseemly elements, and that is because its strength is also its biggest weakness – it has become all things to all men and women.

Laura Bailhache, a 31-year-old solicitor and Oxford graduate, is standing for Ukip in Taunton Deane. She admits that her political allegiance is unusual within her peer group and is refreshingly honest that she doesn't really want to be elected all that much. But Bailhache says the core concerns that Ukip taps into are widely held and she wants to give people with Ukip sympathies the opportunity to signal their support by voting for her. There is some evidence that, on top of the disaffected white Labour vote and reactionary lower middle classes, a third group – those who simply don't vote – are having their heads turned. "Amongst people my age the reaction (to joining Ukip) has been mixed," Bailhache said. "A few people have said to be careful, it is a fringe party, and I think they have got concerns about the racism sticker.

"But I don't think that applied to Ukip at all. I don't think taking control of immigration is racism. But from the other of my friends, more widely, they have just been interested. The stronger thrust from people of my age group is simply not to vote and get involved. If you haven't put your name to any of these people messing it up, at least you can't be blamed.

"But Ukip does seem to be gaining momentum."

So with these various strands of support, is Ukip an unstoppable force?

Ford and Dr Matthew Goodwin, his co-author on a book on Ukip due out early next year, Revolt on the Right, believe not. The incoherence within the party will hold it back, they say. "Ukip have got a real problem," said Goodwin. "Farage has turned it into a party with a broad appeal. A lot of their support now looks like Labour voters and they have the traditional, relatively affluent but insecure, lower middle-class people. But what now? I don't think Ukip actually knows what it wants to be."

Certainly the email conversation revealed today between Godfrey Bloom, the Ukip MEP, and the party's treasurer Stuart Wheeler, a former Tory donor, illustrates an awareness within the leadership of this central problem. The party has no policies to speak of, Bloom admits, and the intrinsic lack of cogency within the party means they are unable to solve that problem.

"As Ukip is a very broad church of membership, I won a Labour seat, we will have many different views on policy", Bloom writes. "Having worked on the defence paper for over one year, it would appear Ukip has more military and naval experts than we have soldiers. Most of them do not agree with each other. It is like herding cats."

Bloom, MEP for Yorkshire and the Humber, adds: "The charm and frustration of Ukip is we have doctors who fancy themselves as tax experts, painters and decorators who know all about strategic defence issues, and branch chairmen, retired dentists, who understand the most intricate political solutions for the nation.

"Our website will have no policies at all on there for 10 years if we adopt a neo-Byzantine approach to formulating them. This means some quite senior party members are going to have to stable their hobby horses." The party should "buy" some policies "off the shelf" from thinktanks, Bloom suggests; why "reinvent the wheel"?

The email also illustrates how the tensions between the old stalwarts of the party and the new membership are coming to the fore. Bloom writes: "We are also attracting new members who bring main party baggage. Focus groups, quotas, even political correctness. We must be wary of listening to these siren voices. We did not get where we are today by following, but leading."

So, Ukip is a party unable to agree on policy, split on ideological grounds, but also divided generationally, with the old stalwarts rejecting the tools of modern politics. Surely all this turmoil and incoherence must be a terrible concern to Farage? No, actually, says one senior source. To come to that conclusion would be to misunderstand what Farage and others at the top of the party want from the party, the source adds. "There is no plan for electoral success and that is because Nigel Farage doesn't want to go to Westminster," the source said.

"He likes his life in Brussels, where he feels he is a powerful man or at least he has power because he can stand up and scream and shout. He wants to continue this role.

"The strategy is to be a pressure group and maintain his position in the European parliament which gives him power and media coverage.

"Farage is supported in this strategy by Stuart Wheeler, who wants to use Ukip as a pressure group to destroy Cameron, because Cameron sacked him from the party and he will never forgive him for this. The aim is to get as many votes as possible so Ukip is a threat and manipulate the agenda.

"What they are really trying to do is drive the agenda of the country."