One of Labour's most senior cabinet ministers today launches a withering critique of the Liberal Democrats, claiming the surge in popularity for Nick Clegg is based on a form of "anti-politics" that provides no basis for government.

In an interview with the Guardian, the foreign secretary, David Miliband, accuses Clegg of trading on a myth – that "Britain has experienced 65 years of failure".

His remarks mark a significant hardening of the Labour position from earlier in the week when cabinet figures such as Lord Adonis were trying to highlight similarities between the two parties.

With Labour now languishing in third place in many polls, the party is trying to stem the Lib Dem surge, and the continuing popularity of its leader, coming into the last fortnight of the campaign.

Rounding on the Lib Dems, Miliband said: "Anti-politics is fine for opposition, but it is not sufficient for government. There is a market for anti-politics, due to the reaction to the sickening expenses scandal, but it is not a basis for running the country … It's not the way to seek a mandate for the running of our country."

Miliband also warned against tactical voting, saying that a strong Lib Dem showing in 100 key Tory-Labour marginal constituencies would be decisive in whether the Conservatives won power. He said voting Labour in such seats was "the only way to keep progressive politics governing this country", adding: "We have said for a long time that David Cameron was not clear what he stood for, and I think Nick Clegg is only clear what he is against. They are riding the anti-politics wave."

A hung parliament was not on the ballot paper, he said, and a Labour vote was "the only way to keep progressive politics governing this country". At one point in the interview he said: "Look, you've punished us enough about Iraq."

He challenged Clegg over his core argument "that we have had 65 years of failure in this country". "If you look at the transformation of the country in the last 65 years it is because of the energy, drive, determination and sacrifice in the Labour party to take on the forces of inertia."

Miliband rejected any suggestion that Gordon Brown should be forced to stand aside as part of a Lib-Lab deal. Clegg has made repeated attacks on the prime minister, saying he is a desperate politician.

"We have chosen our leader. We have got a strong leader and we have got a programme for the future. We are not having Nick Clegg choosing the leader of the Labour party, thank you very much."

Miliband suggested that the public had punished Labour enough over the Iraq war. Asked if there was a serious danger of Labour coming third, he replied: "We have got the fight of our lives, and that is modern politics. It is not tribal in the same way. People are yearning for a more serious conversation."

The remarks are the strongest attack yet on the Lib Dems by a mainstream Labour politician amid signs that the party will next week attempt to shift the focus of the general election campaign away from the TV debates – where Brown has yet to score a victory. It is understood Labour officials have been in constant dialogue with broadcasters, trying to get them to stop obsessing over their own TV debates, and instead cover speeches on significant areas of policy.

Today the Lib Dems continued to demand that the Tories come clean about whether the shadow chancellor, George Osborne, or any other senior member of the party had been involved in briefing journalists from rightwing papers that launched personalised attacks on Clegg.

Danny Alexander, the Lib Dem election co-ordinator, called on Osborne to detail what role he had played, if any, in briefing reporters following Clegg's spectacular performance in the first TV debate last week. "George Osborne needs to come clean as to whether he himself was personally responsible for this negative smear campaign which is now backfiring spectacularly with the voters. It reflects a panic in the old political establishment that they are in a battle for survival."

More than 100 people complained to Ofcom about Thursday's night's second debate on Sky TV, in particular the way in which the presenter, Adam Boulton, raised allegations about donations to Clegg that had been made in the Daily Telegraph. The claims were dismissed as nonsense by Clegg.

In another sign of the shifting political current, another senior Labour figure, the MP Jon Cruddas, hinted at the need to adjust to the Lib Dem surge and be open to tactical voting. He said: "I'm urging everyone to vote Labour, but it's clear that politics is changing before our eyes and we cannot be in denial about that."

In an interview with Jeremy Paxman tonight, David Cameron indicated that a Conservative government would target the north-east of England and Northern Ireland for public spending cuts.