According to conventional wisdom, Ukip has "divided the right". By targeting Europe, immigration and politicians in Westminster, Nigel Farage is tearing off a section of the Conservative base that David Cameron desperately needs if he is to triumph in 2015.

But while it is true that Ukip is currently winning over most of its support from people who voted Conservative in 2010, this actually tells us less than commentators often claim.

In 2010 Labour was at a low ebb, Gordon Brown was extremely unpopular and traditional Labour voters were angry about immigration and the financial crisis. Defining "the right" as 2010 Conservative voters is therefore risky. A lot of those who voted Conservative in 2010 may not have been natural Conservatives at all, backing Cameron despite their misgivings about his party, as a vote against a failed and unpopular Labour government.

A more sensible way of defining left and right is in ideological terms. Ever since Clement Attlee's 1945 Labour victory, British politics has been structured around a conflict over the economy and the proper role of the state.

The left has favoured higher taxation, redistribution and greater state intervention. The right has favoured free markets, low taxes and a small state. This is still a central dividing line today.

Ed Miliband's most celebrated policy announcement called for state regulation of gas and electricity prices, and he has shown a distrust of big business, and a desire for greater taxation of the rich, and greater government help for the less well-off. The Conservatives, meanwhile, retain their traditional faith in free markets and private enterprise.

If Ukip is just dividing the right then we would expect to see Ukip voters falling consistently on the Conservative side of this longstanding divide. But as our chart below shows (based on new data from the British Election Study), the opposite is in fact true.

British Election Study Photograph: Guardian

An average of 71% of Ukip voters agree with five leftwing ideological statements, far above the Conservatives (43%) or even the Liberal Democrats (65%). They are only a little behind Labour (81%).

When Ed Miliband argues that big business takes advantage of ordinary people, employees on zero-hour contracts are being exploited by management, that the rich exempt themselves from the rules that apply to others, and that ordinary workers are not benefitting from a recovery for the rich, Ukip voters agree with him. On these core economic issues, Farage and Ukip do not divide the right. They divide the left.

British Election Study Internet Panel 2014-15

This raises an obvious but also awkward question for progressives. If Ukip's struggling, pessimistic and left-behind voters find these economic messages appealing, why are they supporting Farage, not Miliband?

The problem for Labour is that these voters no longer think about politics in general, or Labour in particular, in economic terms. Labour has encouraged this: New Labour played down traditional leftwing ideology in favour of social liberalism and pragmatic centrism. Now many voters with longstanding "old left" economic values associate Labour more with "new left" social liberalism: feminism, multiculturalism and support for immigration.

Ukip's rise has exposed this division on the left and made it harder to heal. Many of the "new left" voters attracted to Labour by its social liberalism cannot stomach Ukip voters' strong opposition to immigration, which they regard as an expression of ignorance and prejudice, and so refuse to engage with "old left" voters on the economic issues where the two groups share common ground.

Conversely, "old left" voters retain a strong distrust of Labour's middle-class elites, after decades of feeling ignored and marginalised as New Labour chased the middle-class swing vote, and cannot abide lectures from privileged "new left" activists about the virtues of immigration and diversity.

Tony Blair's winning recipe in 1997 was to bury the traditional "old left" Labour ideology, gambling that he could expand Labour's coalition without losing traditional support, as the voters who endorsed it had nowhere else to go. Nigel Farage's rise has made this Blairite balancing act impossible. Ukip has divided the left, splitting the old from the new, and cutting Labour off from struggling voters it seeks to champion.