Dividing lines between the Tories and Labour have historically been sketched out in simple terms. The Conservatives are trusted on the economy. Sometimes the economic medicine may be difficult to take, but it’s prescribed by a safe pair of hands. At the end of June, for example, following two years of coalition, 46% in an Opinion/Observer poll still blamed Labour for the economic malaise. On the other hand, Labour is trusted on public services – most importantly, the NHS. The party may spend a bit freely but its heart is in the right place. Whether doctors at certain NHS Trusts could find them, of course, is another matter…

The trick to an election victory is to keep your lead on the issues that work for you and level up the playing field on those that don’t.

You can play up your own credibility. Throughout the 1990s, Labour worked to convince business leaders that the economy would be safe in its hands. David Cameron took a similar tack on the health service in the build-up to the 2010 election, famously claiming the Conservatives as the party of the NHS. Both strategies were successful, up to a point. But trust is hard to build and all too easy to lose. Labour’s strategy benefited – and Cameron’s may profit too – from external events that undermine confidence in the dominant player.

Trust in Tory economic competence was rocked by Black Wednesday and Britain’s exit from the ERM. Two decades on, David Cameron’s role as special adviser to Norman Lamont during that debacle is still used as political ammunition. Today, Labour’s strongest front is under attack thanks to a series of NHS scandals and this week’s report by Sir Bruce Keogh into the failings of 14 NHS Trusts. The Telegraph, Mail and others have been heavily briefed that the Trust failings happened on Labour’s watch, and Jeremy Hunt has gone on the attack, much to the chagrin of health service leaders.

The Tory strategy is risky. Being linked to the tobacco lobby while opening themselves to accusations of running down the NHS doesn’t play well. But the party would actually be happy for the public to demand a Shakespearean “plague on both your houses”:  as the economy picks up it becomes even harder for Labour MPs to criticise the cuts (they’ve hardly been clear on the matter all along). Meanwhile, the NHS attack leaves Labour fighting desperately in its own back garden, when it should be on the front foot.

“You can’t trust them either” is hardly an edifying thought to impart to the public – but it is effective.