This weekend the UK Independence party (Ukip) will enter a new stage in its strategic evolution. In a television broadcast the party, already the most successful challenger to the main parties in postwar English politics, will launch an invasion of Labour's working-class heartlands. With a growing war chest, Ukip plans to take its message direct to blue-collar communities in red territory. The idea, Leader Nigel Farage has explained, is not complex: "Gun for Labour".

At first glance, this seems bizarre. Conventional wisdom holds that the rise of Ukip has come at the expense of Cameron's Conservatives. So why go after Labour? The reasons are twofold.

First, for some time the so-called clowns of British politics have been pointing to vulnerabilities within the Labour base. One member of their high command told us: "The low-hanging fruit for us are not former Tories, but people who have traditionally and culturally always been Labour."

Bolstered by finishing in the top three in six parliamentary byelections that have been held in Labour-controlled seats since 2011, members talk of exploiting the failings of a "liberal metropolitan intelligentsia, which is uncomfortable among working-class voters, failing to defend their interests, and finds their concerns distasteful".

Second, contrary to much of the coverage that focuses on renegade Tories, the shift actually marks the logical next step in a transformation of the Ukip electorate that is already under way. While the local elections in May revealed that the Ukip insurgency is for real, they also demonstrated its effectiveness within Labour territory. The result was reported as a crisis for the Conservatives, but Ukip gained their best results in the most working-class areas. The campaign saw many deride Ukip for placing billboards in urban areas where there were no elections, but the move was deliberate, intended to sow the seeds for a future invasion into the disgruntled old Labour base.

The message of this future campaign was spelled out by our senior clown: "Who suffers from poor crime? Who suffers from villains who get 17 cautions? Who suffers when the local comprehensive is not effective? Who lost their job in the pub because of a nice-looking girl from Slovakia? It ain't the middle classes. It ain't Ed."

As Steve Fisher, a lecturer in political sociology from Oxford University, has written, in 2013, support for Ukip came as much at the expense of Labour as the Tories, while its roots in working-class areas strengthened as its support surged. In other words, the more Ukip grows, the more it will hurt Labour.

The claim that Ukip is drawing only from the right is one of the big misconceptions in British politics. Our analysis in a forthcoming book on the party reveals that its voters are much more likely to be low-income, financially insecure and working class. They look like old Labour, and since 2010 the Ukip surge has been strongest among these low-skilled, older and blue-collar workers, the exact groups that Labour is struggling with the most. Since its low point in 2009, we find that Labour has made double-digit advances in its vote share among women, the under-35s and graduates, all groups that avoid Ukip. In contrast, Labour has barely grown among men and those with no qualifications, and the party has actually lost ground among pensioners. Among these groups, the Ukip vote has surged by an average of nine percentage points.

An invasion of armies, wrote Victor Hugo, can be resisted – but not an idea whose time has come. While there is lots of talk about a diffuse anti-politics mood, Ukip also articulates a more specific idea: the resentment and anger of the old Labour working-class base.

The Blairites gambled that they could recruit the middle class and retain traditional voters who had nowhere else to go. Half a loaf under Blair and Brown was better than no bread at all under the Tories. For a decade or so the bet paid off handsomely. But it came with a cost. Working-class voters in safe Labour seats felt abandoned by the party that once championed their cause. Turnout slumped, and a sour anti-establishment mindset took hold. Immigration came to symbolise the pathology of New Labour for old Labour loyalists. The former cared more about foreigners than its core supporters, they thought. When they complained, they were attacked as "bigots".

Now the politics of resentment bubbling under the surface of British politics has started to erupt. Ukip strategists are pointing to a coalition of disgruntled social conservatives, who are mocked on the right as "swivel-eyed loons" and dismissed on the left as unbearable bigots. Now screening their candidates, Ukip offers them what the toxic far right couldn't: a populist outlet that does not force them to compromise their democratic principles. Ukip has no grand ideological vision. Its narrative is simple but effective. Enough is enough. No more immigration. No more Europe. No more cosmopolitan condescension from liberal London elites.

The invading Ukip army faces a favourable landscape. Even after three years of austerity and draconian measures to cut inflows, immigration remains second only to the economy in voters' minds. In the last six months the share of voters ranking this issue as important jumped from 19 to 31%, which will surge further as the accession of Bulgaria and Romania nears. Only the economy, voters tell YouGov, will have a stronger influence on their decision at the next election. Public confidence in the Tories to manage immigration has fallen further than on any other issue, but significantly these voters are not switching to other parties. Asked which party they think would handle immigration best, the most popular response is "none of them". And Ukip is not listed as an option.

Such concerns are wrapped in a serious loss of trust. In 2012, trust among the "white working class" in government stood at 23%, equalling the worst figures under New Labour. But when working-class voters were asked who would make the best prime minister, only 22% said Ed Miliband, while 42% said they did not know. This means that for every one (white) working-class voter who supports Miliband's bid for the premiership, there are two who reject all three mainstream leaders. Some may link this to ignorance, but the figures have worsened since 2010, as Miliband has become more widely known.

There are some inside Labour who see the Ukip army assembling on the hills. The MP John Mann argues that Labour must "wake up and get real on immigration", while Blue Labour types hope to translate intellectual debate into genuine connections with workers. But recognising a problem is always easier than solving it. The resentments fuelling Ukip run deep, and have built up over two decades of marginalisation and neglect. Apologies are a start. But there will be no quick fix.

How bad could this get? Labour should look to Europe, where for 20 years social democrats have struggled with the fact that their base is highly vulnerable to radical right invaders. Consider this: during its initial breakthrough period in 1984, the French Front National drew just 8% of its support from manual workers. By 1995, this had rocketed to almost 30%, leaving Le Pen's as the most popular political party among workers. It was the same story in Austria, where between 1979 and 1999 the Freedom party saw its share of the blue-collar vote surge from 4 to 47%, making it the No 1 vehicle for working-class protest. The mechanics of the British system mean that Labour has never been confronted with this challenge. The BNP might have knocked at the door, but when Labour finally kicked into gear it ensured that only few voters let the extremists in. But it is clear to us that Ukip has the potential to come crashing through the house.

Tony Blair won Labour power by winning the hearts and minds of the middle class. If Ed Miliband wants to do the same, he needs to win back the hearts and minds of the workers. An invasion is coming, led by a former stockbroker in a pinstripe suit. Farage is setting up shop in your local pub, Labour, and the locals will like him – a lot. So, what are you going to do?