There is one simple Labour answer to the question of what is wrong with the NHS: nothing, apart from its misfortune in having fallen into Tory hands. Most opposition MPs know that is less than the whole truth but there are clear incentives not to complicate the picture.

The British public reveres the idea of the NHS even when it is disappointed by the reality. Voters also trust Labour much more than the Tories to share that reverence. The collective national memory is hazy on detail but the foundation of a free health service under a Labour government, its deterioration under Margaret Thatcher and rehabilitation under Tony Blair are folkloric.

That is why David Cameron made vows of love for the NHS the centrepiece of his campaign to “modernise” his party and why he must rue his televised pledges not to subject it to “pointless top-down reorganisations.” Labour would happily broadcast those clips on a loop as evidence that the Prime Minister’s pledges are bunk.

The Tory defence is that a budget crisis made drastic reform unavoidable. Change hurts, but the status quo was unsustainable. In other words, true belief in the NHS means willingness to confront long-term challenges. What Labour depicts as duplicitous vandalism, the Conservatives call visionary courage. (Besides, add ministers, the health budget has been ring-fenced to shield it from the ravages of austerity.)

Those arguments, while comforting to Tory ideologues, dissolve on contact with political reality. Cameron is on film saying one thing before doing the opposite. The moment when Prime Minister threw his weight behind a vast and, for most people, incomprehensible reconfiguration of health services, he evacuated his entire stock of trust as a guardian of the NHS. As headlines about staff shortages and waiting times in accident and emergency wards start colonising the front pages, Cameron will struggle to disentangle the mess he says he inherited from the last government from the one he patently made for himself.

Downing Street is braced for a difficult winter. The cold season always produces a spike in demand for the health service and it is already struggling to cope. Meanwhile, Cameron gets conflicting advice about how to respond. Earlier in the year he appeared to share the view of Tory grandees, including veteran health ministers, who counselled that Labour cannot be beaten on the NHS and that a Conservative’s best bet is always to aim for de-politicisation. According to this view, the Prime Minister should treat a winter crisis as a force of nature, appealing to the country’s stoicism and praising the fortitude of hospital workers, as he might do in the event of an earthquake. Labour’s constant partisan attacks might then be made to look tribal and opportunist. Andy Burnham, shadow health secretary, already risks coming across as the NHS doom-monger-in-chief.

The more aggressive approach, preferred by Lynton Crosby, Cameron’s campaign chief, and adopted by the Prime Minister over the summer, is to target Labour’s record. The Tories feel they have won an argument about reckless spending by the last government and want to deploy some of that political capital to insulate themselves from blame for health service misery. The charge is that problems were ignored when the money was flowing and that Burnham, as Labour’s last health secretary, is therefore disqualified from the debate about what to do now that the money has run out.

Jeremy Hunt, the current Health Secretary, has a third way. He aims to position himself as the champion of patients against an unresponsive health bureaucracy. He takes as his model Michael Gove’s approach to local authority schools, casting himself as the raiser of standards and the scourge of complacency in a system that embraces mediocrity and is fixated on only ever doing things the way they have always been done. “Jeremy is always going on about what Michael is doing at Education,” says a senior Department of Health insider. That is the impulse behind calls for more Ofsted-style regulation of hospitals and for GPs to offer more appointments outside normal office hours. Hunt would like to present the problems in the NHS as justification for reform. Labour responds that he is cynically dumping responsibility for the fiasco onto beleaguered doctors and nurses.

The Health Secretary’s strategy cannot repair the damage done by Cameron’s broken promises, although he has found the line the Tories probably ought to have taken in the first place. It is fair to point out that the health service is unprepared to deal with an ageing population whose clinical needs are getting more expensive and whose expectations of care and convenience are conditioned by the service culture of a 21st Century consumer-oriented market economy. Today’s patients are less patient than their forebears. Nor is it controversial to say the NHS budget will struggle to cope with those demands, regardless of who is in power.

Burnham has recognised that conundrum. His answer is an ambitious integration of health, social care and mental health services. In theory, this “whole-person care” mission saves money by deploying resources more wisely, intervening early to prevent solvable problems becoming chronic. It is a sensible long-term agenda but tricky for Labour to sell since it costs money up-front to implement and looks like another dreaded reorganisation.

The easy option would be to bury reform in the manifesto and campaign as if the glory days can be restored simply by freeing the health service from Cameron’s clutches. Labour front benchers insist the message will be more realistic and more nuanced than that. “We are not going to do a rehash of ‘they’ll cut the NHS, we’ll save it,’” one shadow cabinet minister tells me.

That is easy to say now. In a closely fought battle, when hospital wards face closure in marginal seats, there will be irresistible temptation to make promises that can’t be kept. There is an old pattern, followed at various times by all parties, of campaigning as if the NHS can be left alone, realising in office that it must change and then having to confront the anger of voters who feel duped. The Tories have committed that blunder on a colossal scale. The opposition’s advantage is clear. Less obvious is how Labour exploits the situation without making the same mistake.