After Labour’s general election defeat, Jon Cruddas pinpointed George Osborne’s 2012 “omnishambles” Budget as the moment the party lost. “What happened after that is we gained a double-digit poll lead, which hadn’t been earned,” he recalled. “That acted as a disincentive to do all the heavy lifting, to go to all the difficult places after the 2010 defeat.”

Following a Budget that has unravelled even faster than its calamitous predecessor, the danger of complacency is greater still. The Tories’ weakness risks merely diverting Labour from its own.

A one-point poll lead in a recent YouGov survey was enough for some to conclude that the party is on course for victory. If Labour moves further ahead, as voters punish the riven Tories, even more will do so. Yet the lesson of the last parliament was that there really is only one poll that counts. Under Ed Miliband, Labour led by as much as 14 points and ended up losing by 6.5. Voters regarded the opposition as a repository of protest, not as a government-in-waiting.

The clues were there all along. Though Labour frequently led the Tories on voting intention, it trailed them on leadership and economic management - a position from which no opposition had ever won an election. Like a Magic Eye picture, the eventual result was merely hidden. Unless or until Labour leads on one or both of these metrics, it cannot be considered a competitive opposition.

This point was aptly made by David Cameron’s encounter with Jeremy Corbyn in the House of Commons this afternoon. Responding to Iain Duncan Smith’s resignation, the Prime Minister argued that the welfare of the poorest depended on a “strong economy” - something that Labour is not trusted to deliver. Cameron is repeating the attack that so successfully skewered Ed Miliband and Ed Balls. Though voters oppose individual measures such as disability benefit cuts (as they did the bedroom tax), there is as yet no evidence that they trust Labour to manage the public finances or the social security system. Worse, polls show that voters now fear the opposition endangers the nation’s security.

When Corbyn responded to Cameron, he somehow failed to mention Duncan Smith’s resignation and his excoriating criticisms of the government. The most obvious attack line - that the Tories have become too right-wing even for IDS - was not deployed. Instead, in his rambling conclusion, Corbyn repeated his call for Osborne to resign (which he won’t), suggesting that the Chancellor go and do “something else”. The Tories had the satisfied look of a team that has just seen its opponents spoon the ball over the bar.

“It takes a particularly special kind of genius to fuck up today. But, yup, Jeremy obliged,” a Labour MP told me. A shadow minister concluded: “A nice old man dropped against his will into a job, the demands of which overwhelm him. Utterly pitiful and denies the country the opposition it deserves.” Labour MPs were shouting “where is he?” in reference to George Osborne, one noted. “But at the end of this afternoon, they could have been referring to the absence of a semi-competent leader of the opposition.” The missed opportunities will only deepen the determination of some to remove Corbyn. If the 2020 general election appears winnable, Labour MPs will not want to be led by a man they regard as unelectable. The opposition’s epic divisions easily negate those of the Tories.

Though the government’s woes may only deepen (another recession is due sooner rather than the later), the risk for Labour is that the Conservatives recover under a new leader. As in 1990, when John Major replaced the reviled Margaret Thatcher (later winning more votes than any prime minister before or since), a change of prime minister may satisfy the electorate’s desire for change. With George Osborne perhaps irrevocably damaged, Boris Johnson (the country’s most popular politician), Theresa May (the longest-serving home secretary for 50 years) and new work and pensions secretary Stephen Crabb are all plausible candidates.

The “omnishambles” Budget, the botched NHS reorganisation, the Ukip defections, the slowest economic recovery on record - the lesson of the last parliament is that Labour success does not inevitably follow Tory failure. It is one that the party risks being taught all over again.