The first week of the general election campaign had got off to a terrific start for Labour, and previous weekend the party leadership could not contain its delight. A poll giving Labour a four-point lead over the Conservatives, which landed on the night of March 28, was just the morale boost that Ed Miliband’s team needed, going into a gruelling five-week campaign.

One Labour aide says: “All year, the Tories have been told that they are on the verge of a breakthrough. It was supposed to happen at the Budget and it didn’t. We are just a few weeks out and we are competitive.” Even in Tory headquarters in Westminster, where the inmates have been drilled by Lynton Crosby, the Tory election supremo, not to be spooked by variations in individual polls, there was disquiet. A four-point Labour lead would translate into a comfortable Labour majority.

This was not in the script. With a recovering economy, and everything the Tories have thrown at Ed Miliband, how on earth could Labour be showing such a clear lead? “It isn’t looking very good for us,” acknowledged a downbeat Tory MP early last week.

Calls were heard for Prime Minister David Cameron to inject greater optimism into the Conservative campaign, while MPs and ministers began to complain — again — that the Conservatives had locked themselves into an uninspiring strategy focused on talking up the party’s “long-term economic plan” and attacking Miliband. But the previous week, the Tory mood had brightened considerably.

There was no decisive moment in which the campaign turned in favour of Cameron. Don’t expect a sudden game-changer, is the message delivered relentlessly by Crosby. What matters, they are told, is that when voters come to choose (and many voters have yet to decide) they will see that there is a very simple Conservative offer.

The approach is akin to that adopted by the winning side in a particularly hard-fought rugby match. The Conservatives are in the process of grinding out a victory, relentlessly sticking to a game plan and pushing Labour back inch by inch. Although Labour began the week in good heart, the party quickly ran into difficulties when, last Monday, Miliband unveiled Labour’s business manifesto and the backing of a range of pro-EU business leaders. Miliband’s advisers had hoped to turn a potential weakness — Labour’s opposition to letting the voters decide on Europe — into a strength. They also aimed to pre-empt Tory charges that Labour has no support from business.

But the Tories were pleased that Labour had chosen to begin the campaign by concentrating on the economy, where Cameron and George Osborne, the Chancellor, have a big polling advantage. A letter had been circulating among Tory-supporting business leaders and entrepreneurs that endorsed Cameron and raised serious concerns about Labour’s economic policy.

When it was published in The Daily Telegraph last Wednesday, Labour was put in the awkward position of having to attack business. Just 48 hours earlier, it had been seeking to reassure voters that it had some backing from that quarter.

The broadcasters’ election coverage was soon dominated by those who run some of the country’s most important firms attacking Labour’s plans to raise tax and increase regulation. “People understand these are the people who create the jobs that we need and our children are going to need,” says a Tory aide. “And Labour actually wants to raise corporation tax.”

Not surprisingly, the Labour high command takes an entirely different view of the letter. Miliband’s aides told him this was the perfect chance to contrast Conservative closeness to big business with Labour’s policy of restricting zero-hours contracts. “I’m baffled,” confessed a member of the shadow cabinet. “They must be getting the same findings from focus groups as we are, which say the Tories are too close to the rich. And then they push this letter from the rich. It doesn’t make any sense.”

The concern of Ed Balls, the shadow chancellor, and Chuka Umunna, the shadow business secretary, has long been that Miliband and his aides have spent too much time attacking “predatory” big business rather than seeking to win it over.

Now they were both being vindicated. It meant that the Labour team was keen to change the subject. They switched to emphasising Ed Miliband’s personality and sense of humour. A relaxed interview with Absolute Radio that aired on Wednesday was a hit online, and reinforced the belief of the Labour leader’s team that some voters are giving him a second look. His personal ratings were rising.

However, what lay ahead last Thursday night, in a television studio in Salford, was much more testing. The seven-way debate was deadly serious for Miliband and Cameron, each aware that a serious slip could do their campaign fatal harm. For the two nights beforehand, the Labour team had based themselves in the Radisson hotel in central Manchester and rehearsed.

With Alastair Campbell, the former spin-doctor, playing the part of ITV presenter Julie Etchingham, Miliband faced tough questioning. Labour’s aim in the debate was to avoid Miliband being sidetracked into a fight with the minor parties. His advisers and general election co-ordinator Douglas Alexander wanted to “pivot” every attack on Labour back into a condemnation of Cameron and Tory policy.

The Tory strategy — safety first — was simpler. The Conservative leader had to avoid making any mistakes or having a row with Nigel Farage of Ukip, whose voters Cameron needs back. “With seven people on stage,” says a Tory aide, “David had to be careful not to get sucked into the squabbling. Being on the end of the line-up helped in that regard.”

Tory preparations involved Rupert Harrison, the Chancellor’s chief of staff, playing Miliband. Ironically, Harrison is a friend and former student of Lord Wood, the academic who is now one of Miliband’s closest aides. With Crosby, Osborne and No 10 communications chief Craig Oliver closely involved, and US consultant Jim Messina on the end of a phone line, the sessions all emphasised the need for Cameron to stay calm, so that voters would regard him as the natural choice for prime minister. On the night, it worked, even if it meant Cameron looked somewhat dependable and dull. The star was Nicola Sturgeon, the SNP leader, although Miliband also put in a perfectly good performance.

Within seconds of the credits rolling, the Tories were pushing the line that Labour had been outplayed by the SNP. “Sturgeon is a disaster for Miliband,” was the verdict Crosby relayed. Indeed, in the “spin room” after the debate, where party spokesmen compete to try to convince the media that their candidate has triumphed, the Chancellor was extravagant in his praise of the SNP leader’s performance.

Says a Labour rival: “That’s the problem with George [Osborne]. When he thinks he has come up with a clever strategy, he has to wear it on his sleeve. He makes it so obvious. He can’t help himself.” Why should the Tories talk up the SNP? The answer is that if the Nationalists take seats from Labour in Scotland — anything from 20 to 40 — it makes the task of winning across the UK all the harder for Miliband. The Tories have also detected strong resentment on the part of Conservative-leaning voters and potential switchers from Ukip of the possibility of Miliband running England, propped up by marauding SNP MPs.

“We are playing with fire on the union by encouraging the SNP. I hope George knows what he’s doing,” says one Tory MP. Such concerns are unlikely to trouble the Tory leadership this side of the election, not when the Tories have now got Labour where they want it. On the central battleground of the economy, Miliband’s message sounds incoherent. And in Scotland, Labour’s former heartland, there is outright panic as it becomes apparent that the situation really is as dire as the polls have been suggesting.

While both major parties profess themselves satisfied with their progress at the end of the week, and Nick Clegg and the Lib Dems struggle to avoid meltdown, it is the Tories who can take more comfort from the direction of the campaign. There will be many more polls, and no doubt they will show fluctuations. But Tory high command believes the hard grind is paying off. If a Tory victory is not yet secure, Cameron is at least edging closer.

The Telegraph Group Limited, London 2015