It was tempting last year to imagine Labour had the next election in the bag.

The Government’s economic strategy appeared to be a failure. Instead of enjoying economic growth, the country apparently entered a double-dip recession at the start of 2012.

George Osborne, the Chancellor, saw his reputation plummet after a disastrous Budget which gave us the “granny tax” – the abolition of the “age-related allowance”, effectively a tax-break on pensions – as well as an unpopular increase in petrol duty. And opinion polls showed Labour well in the lead.

Labour MPs certainly had a spring in their step as they contemplated the prospect of party leader Ed Miliband entering Downing Street after the next election.

But the mood has changed. It turned out that the economic figures were wrong and the UK did not suffer a second recession last year after all (although Labour’s charge that the economy had “flatlined” was correct – growth was zero rather than negative).

George Osborne’s sins were forgiven, by his own side at least, following a series of U-turns over many of his budget errors.

And while economic growth remains slow – with the International Monetary Fund predicting growth of 0.9 per cent this year – the economy is at least moving in the right direction.

This has allowed the Government to claim the economy is moving from “rescue to recovery”.

To some extent, how voters feel about the economy is more important than what the figures show. And Tories believe that voters are beginning to feel more optimistic about the future. Opinion polls suggest that Labour’s lead over the Conservatives is narrowing.

From around April 2012 most opinion polls put Labour at least 10 points ahead of the Tories.

But that has changed. The latest ComRes poll put the Conservatives on 34 per cent of the vote, with Labour on 37 per cent, the Lib Dems on 10 per cent and UKIP on 12 per cent. That’s a Labour lead of three per cent – the lowest ComRes has shown since last September.

Recent polls from YouGov, another leading polling organisation, gave Labour a lead of between three and seven per cent.

And a poll by ICM, published in mid-July, found that Labour and the Conservatives were tied, each backed by 36 per cent of voters.

Every other poll has given Labour some sort of lead. And in any case, opinion polls may not be a good guide to the precise share of the vote each party would receive in a real election.

But there has been a clear trend over a period of time for the gap between the parties to fall.

One potential explanation for this is that support for UKIP is beginning to ebb away (although many polls still show it is more popular than the Lib Dems).

UKIP leader Nigel Farage likes to point out that some of the party’s support comes from former Labour voters, not Tories – a point echoed by Conservative chairman Grant Shapps.

But in practice, many Farage fans are indeed former Conservative voters. If they are returning to the Tory fold it may help to explain why the Conservatives are closing on Labour. This would actually suggest that Ed Miliband’s lead was never quite as healthy as it looked even when Labour was roaring ahead.

Some of UKIP’s support comes from people who are fed up with the major parties and want to send them a message. In other words, it’s a repository for protest votes. And while that can lead to success in by-elections, European elections and opinion polls, protest voters tend to return to one of the main parties when a general election comes around.

So if Labour’s lead in opinion polls was partly due to the growing popularity of UKIP, it suggests that lead may always have been exaggerated a little.

But there may be other reasons for the revival of Tory fortunes. The economy, as mentioned, could be one.

The party’s tough stand on benefits, although highly controversial and bitterly opposed by campaigners who argue it victimises some of the most vulnerable people in society for political purposes, is believed by the Tories to be a vote winner.

The focus on failings in the NHS may also be having an effect.

Health Secretary Jeremy Hunt was condemned by Labour MPs when he argued, in a personal attack on Labour Shadow Health Secretary Andy Burnham, that the previous government’s focus on targets had damaged patient care.

But while Conservatives may never succeed in David Cameron’s aim of replacing Labour as “the party of the NHS”, they might at least succeed in casting doubt on Labour’s competency when it comes to running the health service – taking away, or undermining, one of the positive reasons for voting Labour, even if they cannot transform it into a positive reason for voting Conservative instead.

The truth is that we can never know for sure why voting intentions change.

As YouGov’s Anthony Wells explains, on his blog ukpollingreport.co.uk, simply asking people doesn’t work.

He wrote: “If you ask people who gave a different answer three or four months ago if they’ve changed their mind many won’t realise they have.

“If you ask why to those who have consciously changed their mind you get lots of don’t knows, general grumbles and some reasons that may be genuine causes, or may be post-hoc rationalisations for complex decisions we probably don’t even understand ourselves.”

There’s also a tendency to assume poll results back up our personal political views. Supporters of a referendum on Europe might imagine voters are returning to the Conservative Party because David Cameron has offered one, for example.

One interesting result to come out of the latest ComRes polling is that voters don’t much like coalition government, with 73 per cent saying they “prefer one party to win an outright majority at the 2015 general election than have another coalition government.”

And asked to look back at the 2010 election, 29 per cent agreed that “Britain is better off with a coalition government than it would have been if either the Conservatives or Labour had won the last General Election” – with 57 per cent disagreeing.

This could potentially spell bad news for the Lib Dems. If a hung Parliament looks likely, the two major parties will be able to argue that a vote for Nick Clegg is a vote for another coalition (even though the way the vote is shared out between Labour and the Tories is the most important factor – the Lib Dem share of the vote rose by only one per cent in 2010).

It would be an historic victory if David Cameron won an outright majority in 2015 – and it’s still hard to see how he could. He’d need to win more than the 36 per cent of the vote the Tories received in 2010. And no sitting Prime Minister has increased their party’s share of the vote since 1974. Even Margaret Thatcher in the 1983 election, after the Falklands War, saw her share fall by 1.5 per cent from the 1979 election.

The party conference season this summer will be crucial in determining whether Labour can regain momentum and end Tory dreams of making history.