An overconfident Tory campaign has lost its energy and momentum, with Ed Miliband proving resilient under pressure, according to Labour’s US political guru. David Axelrod, a former campaign strategist for Barack Obama and now Labour’s hired hand, said the Tories “keep pushing buttons that are not working”, which is leading them to “lurch from tactic to tactic”.

In an interview with the Guardian, Axelrod said Labour’s principal opponents had entered the year with “a kind of cocksuredness predicated on the belief they could caricature Miliband and caricature the Labour party” and “their failure to do so had left them “increasingly panic stricken”.

Axelrod argued that the Conservatives’ sudden switch from talking about Trident and the character of the Labour leader to making sizeable spending promises about the NHS and other policy areas was being noticed by the electorate.

He said: “British voters are catching on that these blandishments that David Cameron is offering have an expiration date on them and the expiration date is 8 May. As soon as he is returned to office, he will return to the very same policies that have conspired against working families for the past five years.”

Axelrod’s combative assessment comes at the end of a week in which the continuing stream of good economic news still did not push Cameron past Miliband, even if fresh polls by Lord Ashcroft suggest that an avalanche of support for the Scottish National party will see the Labour high command in Scotland defeated.

The growing likelihood that any minority Labour government might be dependent on a big phalanx of SNP MPs prompted Cameron to claim Miliband would be forming a “coalition of chaos” that would bankrupt and break up Britain. Miliband again ruled out full-scale cooperation with the SNP and said the party leader Nicola Sturgeon’s agenda remained a second referendum on independence for Scotland.

David Axelrod: ‘British voters are catching on that these blandishments that David Cameron is offering have an expiration date on them.’ Photograph: Mike McGregor/the Guardian

Axelrod argued that a vote for the SNP was like “a bank shot” – a sporting metaphor implying risk.

There has been widespread criticism inside and outside the Labour party that Axelrod – who is said to be earning £300,000 to give his advice – has not been fully engaged. But he laughed this off and said he was in constant touch with key party strategists and had visited the UK four times. He is due to return to Britain on 26 April for the campaign run-in.

Axelrod, hired last summer, was a key figure behind the election of Obama as an Illinois senator in 2002 and in two presidential elections. His chief skill is shaping an overall message for a campaign.

He believes the Conservatives – who are in part advised by his own Democratic campaign colleague Jim Messina – made a fundamental error in underestimating Miliband. “Obviously it is a battle and we are up against a lot of organised power and incumbency,” he said. “But I think what has happened is that Ed has been discounted so long by the media that when the spotlight was finally thrown on him he gained from that.

“His resilience has become a great story in itself. People have counted him out and counted him out, particularly the media, but he believes in what he is doing.”

Axelrod said most voters did not pay attention to the detail of politics in the way that the media and insiders did, instead getting broad impressions and focusing when they have to.

Brushing off criticisms of Miliband’s performance, he said the Labour leader was coming through when it mattered, at the end of a campaign. “That is when people really need to shine and Ed has risen to that challenge. The public are matching what they are seeing with their own eyes against a media cartoon depiction and they are pleased with what they see.”

Labour leader Ed Miliband and David Axelrod talk at Labour party headquarters. ‘His resilience has become a great story in itself,’ Axelrod says. Photograph: Stefan Rousseau/Press Association

Axelrod has always argued from his experience of more than 150 mayoral, senate and presidential campaigns that the campaign itself can be a proving ground for a candidate’s strength. He said: “What I have seen in my work is that campaigns are tests. They are endurance tests. You are auditioning for a very tough role and the way you deal with adversity and the way you deal with pressure and challenges gives people clues as to what kind of leader you’ll be, and Ed has benefited from that.”

He argued that the Tory attempt to character-assassinate Miliband had been an error, referring particularly to the way in which the defence secretary, Michael Fallon, had claimed anyone prepared to “stab his brother in the back” to gain the Labour leadership was prepared to stab the nation’s security in the back.

“Plainly that backfired because they got off of it pretty quickly. People are worried about their own family, they are not worried about Ed’s family. They worry about whether their kids are going to make it in the future. They want to hear some solid ideas. All of this other stuff appears like a sideshow and they notice it is a sideshow. It appears desperate and it was desperate.”

Axelrod said the most surprising aspect of the campaign so far had been the extent to which the Conservatives had lurched around. “At the beginning it was ‘Labour is profligate’. That did not work. Then it was a personal attack on Miliband and that did not work. The next attack was the £3,000 tax increase and then that proved not to be true. Now they have gone full circle and are making 11th-hour promises they have no way of funding since their budgets calls for the most extraordinary austerity.”

Axelrod argues in relation to presidential campaigns that public opinion is not looking for a replica, but a remedy – and so in this election he is happy to emphasise the gulf between the two potential prime ministers on offer in Britain.

“The truth is there is a fundamental difference in the philosophy between these two leaders. David Cameron honestly believes that if the people at the top do well, all Britain succeeds. Ed Miliband truly believes Britain succeeds when working people succeed. This Cameron camouflage at the end of the campaign does not obviate the fact that his is very much a top-down philosophy.”

He ended by warning about the “fog of fear” in the final days of the campaign and warnings of chaos from the Tories. “Fear is always the last refuge of a panicky campaign. As we get closer to the election, we will hear a lot of that.”