Even Nigel Farage accepts that Ukip is nearing the limit of its appeal to disaffected former Conservatives. The party's ability to grow depends on its ability to take votes from Labour – hence Farage’s bluster about putting his tanks on Ed Miliband's lawn. There are signs that it is working, too. Before 2013, Ukip took only one Labour vote for every nine they took from the Conservatives. But since January 2013, Labour has lost six voters to Ukip for every nine that Ukip has taken from the Conservatives.

The rebranding of Ukip has taken three forms. The party, for so long hopeless at campaigning, has learned from the Liberal Democrats about the importance of relentless campaigning in by-elections – and sheer opportunism, as with the recent attacks on Labour’s left flank. As a disproportionate number of by-elections this parliament have occurred in northern seats, Ukip has become well versed in attacking Labour.

Ukip has also shifted its policies – and especially which ones it chooses to emphasise. Privately, many leading figures within the party do not consider the NHS viable in its current form. But the party has chosen to oppose not only the current government’s reforms, but also New Labour’s. "The NHS is a battle for another day," one party source tells me, reflecting how Ukip has decided that opposing all reform of the health service is the most fertile source of votes. Such populism is detectable in other policies, too. Ukip has flirted with the "wag tax" (until Farage shot down the idea) and party figures are even floating renationalising the railways. "I do think that we should be considering, and there should be an open debate at the moment, whether we should have nationalisation or [run the railways] through an organisation like a co-op," Steven Woolfe, the party’s financial affairs and migration spokesman, recently told me.

Finally, there has been a change in personnel in the party. To undermine the caricature of the party as one of disillusioned shire Tories, Ukip has pushed forward figures like Woolfe, Diane James and Paul Nuttall, who each have working class credentials. It is a long way from the last election when the party was led by Lord Pearson, an Eton-educated life peer, although the defection of two privately-educated Conservative MPs could yet undermine Ukip’s appeal to Labour supporters.

Labour has decided that its general election attack on Ukip will be to present the party as "the Tories on speed," as a shadow cabinet member puts it. "We’ve found that’s what works best." But Labour’s problem, as a new Comres poll shows, is that the electorate don't quite agree. Asked to put leaders and parties on a left-right scale of 0-10, they put Farage (6.59) and Ukip (6.61) to the left of David Cameron (6.81) and the Conservative Party (6.91).

This is very significant. For some voters who consider the Conservatives too right-wing to support, Ukip is a more palatable option. Though Ukip is significantly to the right of the average voter, the poll suggests that the party’s repositioning – away from libertarianism and towards the populism of a leftist and rightist bent favoured by the most successful anti-immigration parties on the continent – is proving successful. Ukip’s judgement is that many former Conservatives fuelled by resentment of David Cameron are now in the party to stay, so it can shift leftwards to try and broaden its appeal without alienating them. Labour has been warned.