Labour is likely to be outspent by the Conservatives by a factor of three to one in the general election, the party’s election boss has admitted, but insists it can still win the tightest battle in generations through an intensive ground war built around local party activism.

The message comes before a rally in Manchester on Monday where Ed Miliband, the party leader, is due to mount a ferocious assault on Tory plans for the health service.

The rally is seen as the starting point of a campaign that will see Labour facing hostility in large sections of the national media and heavily outspent by the Conservatives, and so highly reliant on its superior organisation on the ground to bring out its vote in key marginals.

Douglas Alexander, Labour’s election coordinator, writes for the Guardian: “The air war still has its place but it is on the ground where this election will be won or lost. Anyone who, like me, spent last summer in church halls and village halls, high streets and doorsteps across Scotland will understand this demand for dialogue.

“The Tories may be able to outspend us by as much as three to one, but on the ground, in the key seats, we aim to outnumber their diminished and demoralised activists by the same margin as we fight this election conversation by conversation.”

His article comes after David Cameron on Friday unveiled the Conservatives’ first election campaign poster on a visit to Yorkshire with the slogan “Let’s stay on the road to a stronger economy”.

The prime minister said: “I say that we should stay on this road because I’m absolutely clear about what the destination should be, about what more we can do for our country over the coming five years.” The alternatives were “frankly … disastrous”, he said.

A claim on the poster that the coalition had “halved the deficit” drew attacks from Labour, and critics said it was only true if measured as a proportion of national income. In cash terms, it has only been reduced by about a third.

Alexander, writing for the Guardian’s Comment is free, also discloses that the Labour election campaign is expected to be built round a spine of five pledges. The first two, already announced, cover the deficit and curbing immigration – two natural Tory subjects.

He reveals that the remaining three will be based on more familiar Labour issues: the future of the NHS, living standards, and the future for young people.

The whole campaign theme is constructed around an economic argument that since the crash the Conservatives have failed to reform the economy so that it tackles inequality and delivers for working people.

Alexander’s claim that Labour will be outspent by three to one is based on estimates of how much party candidates will spend locally as well as national election spending.

In the 2010 election campaign the parties spent £31.1m, with the Conservatives accounting for 53% of this total (£16.6m). Labour spent 25% (£8m) and the Liberal Democrats 15% (£4.8m). But the Tories have raised £78m over the past four years, making the imbalance in incomes larger than 2010.

Alexander admits that with little more than four months to go the election is wide open. Setting out the big challenges facing the Labour campaign, he writes that the established parties face an electorate that, as is the case in the rest of Europe, is no longer apathetic but angry.

He warns: “The crowd may not be leaving the stadium, but they may be turning towards other teams who are playing a new and dangerous game.”

Alexander writes that in spite of what he describes as “the rise of the rest in 2014”, the battle for Downing Street in 2015 “remains a binary choice between the change we need and can deliver with a Labour government – or more of the same failure that we have suffered under this Tory-led government”.

Labour believes a shift in local activism from “no campaigning” to “average campaign intensity” leads to an identifiable increase in the party’s share of the vote. Its research shows that if Labour managed to contact 30% of voters in a seat in 2010, its share of the vote rose by over 5%.

By mid-November, in its key seats, Labour had contacted over 15,000 voters per constituency, 21% on average. It hopes to have pushed that figure to 25% by the end of the year, and take it higher in the months before polling day on 7 May.

Faced by voter anger, Alexander maintains that the answer does not lie in making outlandish pledges but instead promising clear and credible change.

His remarks come a day after Ed Balls, the shadow chancellor, argued that George Osborne’s autumn statement had given Labour the chance to become the party of the centre ground, resisting extremist and ideological plans to reduce spending on public services to the level of the 1930s.

Balls has been helped in that attack by the surprisingly virulent criticism of the chancellor mounted by two of the Liberal Democrats’ leading centre-right politicians, David Laws and Danny Alexander.

In what is likely to be the first battle of the long election campaign, Osborne remains confident that he has made the right call and voters are more worried by the deficit than the size of the state.

He argues that if Balls is to vote for the fiscal rules set out in Osborne’s new code on fiscal stability, the shadow chancellor will have to give details of the big tax increases he will have to impose to get the current account into balance.

The average of polls shows Labour with a three-point lead, sufficient to secure an overall majority of 30, but these figures do not take into account specific factors, notably the surge in support for the Scottish National party – now on course to win anything between 25 and 40 seats.