Politicians like dressing up: it is one of the many things they have in common with children. Prime ministers wear flak jackets on visits to military bases to exude strength. For chancellors, the costume of choice is Bob the Builder. Nothing says "getting the job done" like a hard hat and high-visibility jacket. In the days before last November's autumn statement on the economy, George Osborne - usually sparing in his TV appearances - kept showing up on construction sites looking as if he had leapt from the cabin of a JCB to share some thoughts on infrastructure investment.

Nick Clegg also likes to get his overalls on for the camera, admiring the craftsmanship of industrious youths freshly redeemed from the dole by a government-funded apprenticeship.

The logic is simple. Most people, most of the time, pay no attention to politics. When they do, they forget what is said. Promises are presumed worthless. Voters do not want to hear boastful claims about what politicians think they have achieved. The attitude, as one Downing Street strategist puts it, is "show me, don't tell me".

That adage, borrowed from the world of marketing, is well understood by David Cameron, whose only job outside politics was in public relations. His campaign to "decontaminate" the Tory brand in opposition generated the most famous fancy-dress outing in recent political history: the Conservative leader as polar explorer, fretting about climate change with sympathetic huskies.

Toxic stew

Now Cameron probably wishes he could dress up as a doctor and be filmed performing life-saving surgery. He urgently needs to look like someone who cares about the NHS. Before the election, he promised to protect the health service from painful budget squeezes and disruptive bureaucratic reconfigurations. He is inflicting both, thus reinforcing a lingering suspicion that the Conservative Party's instinct is to vandalise a treasured national institution.

This is an extraordinary lapse for a Prime Minister who is respected even by his enemies as a deft manager of public opinion. The failure is twofold. First, Cameron did not pay adequate attention to the toxic stew that Andrew Lansley was cooking up at the Department of Health. Then, as a rancid stench filled the air, the Prime Minister, reluctant to perform a U-turn and confident in his powers of persuasion, thought he could sell it to the public anyway. He can't.

The painful truth about voters' trust in Cameron where the NHS is concerned is that it rested not so much on what the Conservative leader had said but on the way his message was made personal by the experience of caring for a severely disabled son and by the agony of losing him.

Politics is a callous business. Any benefit of the doubt afforded to a grieving father has been submerged in the old assumption that Tories just don't love the health service enough. Insisting that they do cannot change things. Show us, say the voters, don't tell us.

Labour cannot believe its luck. Public faith in the party's dedication to the NHS is automatic, so Ed Miliband can attack government reforms without pausing to discuss what, if elected, he would do differently. "We don't have much of a policy," concedes one shadow minister, "but on health, I don't think we need one."

That might be true. There is, however, a risk that the pleasure of pummelling the Tories on the NHS will seduce Labour into deferring trickier conversations about public-sector reform. Andy Burnham, shadow health secretary, presents himself as the scourge of market forces in health care. That chimes with public concern; it also implies a repudiation of Labour's own health policy under Tony Blair. Miliband is more nuanced, saying limited competition is acceptable within the NHS "internal market" but pernicious if the private sector is allowed to run rampant.

This is a holding position while Labour works out what it feels more generally about private companies running public services. I regularly ask people close to Miliband where he personally stands on this question. He doesn't know yet.

That indecision expresses a paralysing fear of party disunity. The choice earlier this year to rule out a blanket reversal of coalition cuts triggered a spasm of indignation from the left and the trade unions. Miliband is in no hurry to provoke another one by flirting with Blairish notions of public-sector reform. But having accepted that budget discipline is the touchstone of economic competence, Miliband has to show a capacity to make painful choices and imagine ways to get more value for less money spent on the public sector.

That needn't mean naming the services he would cull or spelling out a detailed reform package years before an election. Premature manifestos just get monstered by the enemy. It does mean deciding on priorities and communicating them in ways that stick in people's minds. This is Miliband's "show me, don't tell me" challenge. Saying you are trustworthy doesn't make people trust you, in much the same way that telling people you are funny doesn't make them laugh.

“No huskies" rule

Since winning the leadership, Miliband has stuck to a "no huskies" regime, eschewing what he sees as the insincere gimmickry that defined Cameron in opposition. He is right to be wary. Inauthenticity is always found out in the end. The problem is that, for want of alternative devices, the Labour leader's message is trapped in speeches and newspaper articles that bounce off the wall of public consciousness without leaving a mark.

The battle over the government's NHS reforms is a gift to the opposition - and a trap. Labour can score easy points telling people that Cameron is not to be trusted with the health service because that is what they think already. That doesn't help when it comes to communicating all the many things the public doesn't know about Edward Miliband. Already the void is filling up with doubt and disdain. The moment when the Labour leader could tell people what he believes and expect them patiently to listen has passed. It's show time.