Conservatives will gather in Manchester for their annual conference in high spirits – but any hint of triumphalism could be their undoing.

They have two reasons to be happy. First, the economy is on the up. They feel they can point to Labour’s prophecies of doom and say that the opposition has been proved wrong.

Predictions that public spending cuts would choke off growth have not come true and there was no need for a “plan B”, as Shadow Chancellor Ed Balls claimed.

Secondly, they are heartened by Ed Miliband’s speech to his own party gathering in Brighton.

Mr Miliband pleased his activists by announcing a series of tough measures including a freeze on fuel bills, threats to seize unused land from developers failing to build homes (actually a policy he announced earlier this year but one he highlighted in his speech) and reversing Government plans to cut corporation tax. Labour would instead cut business rates for smaller firms, he said.

But Conservatives believe Mr Miliband has planted his flag firmly on the political left – while parties can only win elections from the centre.

So what could go wrong? The biggest difficulty facing the Tories, paradoxically, is the upturn in the economy.

Labour’s focus on the “cost of living crisis” has struck a nerve. If David Cameron or his Chancellor, George Osborne, sound too ecstatic about economic growth then they risk confirming the Labour argument that they don’t live in the real world and don’t care about the many people on low or medium incomes who feel poorer, rather than richer, than they used to.

So Conservatives in Manchester will be at pains to make it clear that they too have plans to help people who are suffering financially, and understand the need to ensure the benefits of the recovery are shared with everyone.

But they also feel Labour is in danger of exaggerating the extent to which people have suffered during the past three years, especially given that low interest rates have kept mortgage payments low.

One Conservative MP said: “One of the tragedies of opposition is you always wish ill on the Government which means you end up wishing ill on the country in fact.

“They were wrong about the flatlining, they were wrong about plan B, they’ve been wrong on everything.

“All they can say now is that the economy is recovering but it’s only helping the rich.

“But it is a counsel of desperation.

“Because if you have a job this recession has hit you to be sure but it’s not been the years of misery that Labour talks about.”

Ed Miliband’s speech, described as a “lurch to the left” by some national newspapers, has also given Tory MPs hope.

One said that it was a good speech in many ways, delivered well, which undoubtedly strengthened Mr Miliband’s position in his own party.

But they believe he has “nailed his colours to the mast” as a left-wing politician with a left-wing agenda.

Even if this is true – and it’s hard to deny, with left-wing Labour MPs expressing their delight with their leader’s willingness to reveal his true colours – it remains to be seen how much of a handicap it will really prove to be with the public.

After so many years in which politicians of all parties staked out the centre ground – and often the centre-right ground – voters may be ready to consider a change.

But Tories believe the charge that Miliband is a left-winger or a socialist will scare many older voters who remember the 1970s.

One Conservative said: “He is guaranteed to be the leader at the next election - that’s clear after a speech like this.

“But elections are won from the middle ground.

“They are not won from the hard right or the hard left. It’s bizarre.

“It’s not a Socialist mantra, its almost Communist. It’s what the Russians used to do – regulation of almost everything.”

There is also a suspicion that the Labour’s near-silence over the summer suggests all is not well within the top ranks of the Labour Party.

The leader cannot be on the television screens every day – and everyone needs a holiday.

But one might have expected Mr Miliband’s colleagues, such as Ed Balls, the Shadow Chancellor, or Yvette Cooper, the Shadow Home Secretary, to have stepped in to the gap and kept the assult on the Tories going.

They didn’t, and the perception grew that Labour was failing to campaign, for which Mr Miliband took the blame.

Ed Balls’ decision during the Labour conference to re-write party policy and cast doubt on Labour’s commitment to high speed rail is also seen by Conservatives as a sign that he is putting his own personal ambitions ahead of supporting Mr Miliband.

But Conservatives admit they have problems of their own.

The party desperately need to find a way to win back support from women, after a poll by the Mumsnet website found Labour is now 13 points ahead among female voters – who see Mr Cameron as too posh.

The poll, carried out by Ipsos Mori on behalf of the website, found that 42 per cent of women would back Labour, compared with 29 per cent who would vote Tory.

Conservatives have also failed to convince enough voters from ethnic minority communities to back the party, even though they believe ethnic minority voters are disproportionately likely to back “Tory values” of hard work, self-improvement and support for families.

And then there is the threat from UKIP. While UKIP leader Nigel Farage argues that support for his party comes from every section of society, including former Labour voters, Conservatives believe they have to encourage their own supporters who have defected to UKIP to return to the fold.

The good news, according to Tory politicians, is that David Cameron has learned to involve colleagues in policy making and political strategising. Power is no longer concentrated solely in Number 10 and Number 11, the Chancellor’s home.

A lot will depend, as always, on David Cameron’s own speech on Wednesday.

But Tories heading for Manchester are optimistic. They recognise that in many ways, Ed Miliband had a good conference – but they suspect it may help them more than it helps him.