The opinion polls are up-and-down day-in-day-out at the moment, making it almost impossible to say with any confidence whether we are firmly in hung parliament territory, or whether the most likely result is still a Tory victory at the coming general election. But one thing is beyond doubt: the last six months has seen a substantial narrowing in the Tories’ opinion poll lead.

In October 2009, the Tories were polling at around 42%, Labour at 28% – a convincing Tory lead of 14%. Last month, the Tories were at 39%, Labour at 31%, a 3% swing from the Tories to Labour. The Lib Dems have stayed steady at 18-19%.

Most Tory supporters have been left bewildered by the rapid turnaround. Just a few months ago, they were brayingly confident of enjoying a commanding Commons majority. Today, they’re nervously considering whether they will even be the largest single party. For the record, my guess is they will be, and that the Tories will emerge with a slim majority – think Harold Wilson in 1964 (but with a much larger Liberal presence, of course).

But the question remains: how did David Cameron’s Tory party lose its poll lead? How, given Gordon Brown and Labour’s appalling screw-ups over the past five years, is it possible that the Tories cannot be confident of victory in May?

The Financial Times’s Philip Stephens has identified the moment he thinks the Tories, drunk on their own sense of entitlement, botched it:

The Conservatives made their big mistake last autumn when Mr Cameron joined his shadow chancellor George Osborne in promising a new age of austerity. Hitherto the Tory leader had defined himself as a post-Thatcherite politician, eager to heal the society whose existence the famous Lady had so infamously denied. Now he took up her ideological cudgels against big government with a promise to slash and burn across the public sector.

I think Philip is right. In fact, it wasn’t so much what George Osborne said, as the seeming relish the Tory party (and its right-wing cheerleaders at ConservativeHome) took in stressing the need for deep, urgent public spending cuts. For all that the shadow chancellor deployed the refrain, “We’re all in it together”, the impression left was that ordinary British people were going to be left picking up the tab of the excesses of the Tories’ banker friends.

If Boy George’s 2009 speech was indeed the trigger moment for the Tories’ declining popularity, there is some irony. He owes the fact that he retained his current position as shadow chancellor (rather than ceding it to the vastly more popular and experienced Ken Clarke) to his 2007 conference speech, when he infamously pledged to cut inheritance tax for 3,000 of the country’s wealthiest households to acclaim within the Tory party.

What looked like tactical genius a couple of years ago continues to look like a strategic blunder today, teeing up lines such as Nick Clegg’s to the Lib Dems’ spring conference on Sunday:

The Conservatives may want tax cuts for millionaires. We will deliver tax cuts for millions.

The Economist’s columist Bagehot remarked last month of the Tories’ gloom-mongering:

… at the moment the Tory prospectus is overly dominated by promises to prevent some bad things (high interest rates and so on), and, conversely, promises to make some other “bad” things happen—pay freezes, reductions in some welfare benefits and, implicitly, public-sector job losses and tax rises. Oh, and because of the new emphasis on transparency and wiki-government, voters will be able to watch the cuts happening online.

In his first year, David Cameron’s emetic promise was to “Let sunshine win the day“. Tuesday, 6th October, 2009, was the day George Osborne sent in the clouds.