Conservative attacks on Ed Miliband are failing to turn wavering voters against the Labour leader, the former Tory treasurer and influential pollster Lord Ashcroft has said.

There has been too much emphasis from the Tories on the opposing leader’s weaknesses Lord Ashcroft

Rather than “crumbling” under fire, Miliband had won support by showing “a good deal of resilience in the face of some rather unseemly attacks”, Ashcroft said.

Writing in the Independent, the Tory peer said focus group findings showed that voters who had previously tended towards Labour but preferred David Cameron as prime minister were now plumping for Miliband after observing that he had done “better than they expected” in the general election campaign.

Ashcroft suggested that Conservatives had made a mistake in opting to focus their campaign on Miliband’s supposed weakness as a leader, rather than setting out a positive vision of the future under a Tory administration.

But he stressed he was not blaming Australian elections guru Lynton Crosby, who has played a major role in shaping the Tory campaign.

Alberto Nardelli rounds up this week’s opinion polls and how they translate into seats in the Guardian’s projection. Guardian

Crosby could only work with the materials he was given and had brought “discipline, organisation and consistency” to the Conservative operation, Ashcroft said.

He said his polling suggested Labour was “winning the ground war” of contacting voters in person and through leaflets and phone calls.

“If the blue army is being outgunned, that is not a matter of logistics, but because it lacks recruits,” he said. “And that would not be surprising for a party that has been unable to reach very far beyond its core support for more than 20 years.”

“If the Tories were expecting the contrast between Cameron and Miliband to work as a kind of secret weapon, they are still waiting for it to take effect.

“There has been too much emphasis from the Tories on the opposing leader’s weaknesses (or, in this case, the deals he may or may not do to get himself into office), which suggests to voters a party that can’t have much to say for itself.

“But, if after five years in government, the Conservative party’s policy advantage is confined to the economy and the public finances – crucial areas, but not, as far as voters are concerned, everything – that is hardly the fault of an Australian consultant.

“Rather than relying on the identity of their leader and the risks of change, the Tories over the last five years ought to have laid the foundations for a campaign in which they could talk confidently about their plans for public services and to describe a Conservative vision of opportunity and prosperity for all.”

Ashcroft had some cheering news for the Liberal Democrats, who he said were “doing best in the marginals, holding on to more seats against Labour and (particularly) the Conservatives than they would with a uniform swing”.

It was still not too late for a swing to the Tories that might keep Cameron in 10 Downing Street, he said, but added: “If undecided voters make up their mind to stick with the Tories, I suspect they will do so very late.”