Most election campaigns have bad days, a brief panic provoked by a bad poll or grim headlines. Thursday 9 April may not be the Tories’ only day when they wobble, but let’s hope there is never a nastier one. At the least, the party should learn the lesson of the way the deeply personal and dishonourable attack by Michael Fallon, the defence secretary, on Ed Miliband has boomeranged right back. Mr Fallon’s article in the Times, and his robust defence of it on the airwaves all morning, even embarrassed his fellow Conservatives. Both David Cameron and Philip Hammond, the foreign secretary, avoided endorsing it. It has exposed the uncertain tenor of the Conservative attack, which wavers between calling Mr Miliband first too ruthless and then too weak to be prime minister. And it was wrong in substance too. If Labour governed with Scottish Nationalist support, the party would not be forced to abandon the renewal of the Trident nuclear deterrent, any more than the Conservatives were forced to in negotiations with the Liberal Democrats in 2010. On the contrary, a minority Labour government could, if it wished, renew Trident, relying on Tory votes.

A fortnight ago, Ed Miliband told Jeremy Paxman expressly that he would not trade Trident for SNP support

At the start of the year, many Tories were confident that despite months of level-pegging in the polls, once the campaign began in earnest they would pull away from Labour. But so far there is no sign of any such advantage outside the range of normal statistical error. In the latest polls, Labour has crept back in front. The personal ratings of the Labour leader, which had been stuck deep in negative territory, have been showing a steady recovery since the televised debates began a fortnight ago. In one poll he has overtaken Mr Cameron, and all the ratings are going in the right direction for Mr Miliband, and not at all in the direction anticipated in the Tory war room. Then on Wednesday, the Conservatives were wrong-footed by Labour’s pledge to end non-dom tax status, a move backed by some prominent Tories, in the columns of the Financial Times and, according to one overnight poll, by 77% of the electorate.

The extraordinary attack by Mr Fallon speaks volumes about morale in Tory HQ. To equate Mr Miliband’s readiness to challenge his brother for the party leadership with an unprincipled appetite for political power is both offensive and silly. To question a political leader’s loyalty to his or her country demeans Mr Fallon. It also inflicts another blow to political discourse. It is bad enough that the election is being conducted as a series of heavily stage-managed encounters between party leaders and handpicked loyalists. To stoop to the kind of calibrated hysteria of the Fallon attack marks a new and miserable low.

What is even more extraordinary is that, as the Lib Dems are pointing out, within the terms of the coalition agreement of 2010 the Conservatives themselves conceded a review of options short of a full-scale replacement for Trident, and accepted a delay until 2016 of the so-called “maingate” decision – the irreversible commitment to replace the continuous deterrent. A strategic defence review will be the first priority of any incoming defence minister, regardless of party. The renewal costs of Trident, which are now estimated at about £23bn, will have to be weighed against the impact of further austerity on conventional forces that are already strained to the limits of functionality. Labour has made the commitment in principle to continuous at-sea deterrence that currently requires a four-submarine fleet, but senior opposition figures, including the former defence secretary Lord Browne, and the Lib Dem elder statesman Menzies Campbell, anticipate a further inquiry. A fortnight ago, Ed Miliband told Jeremy Paxman expressly that he would not trade Trident for SNP support.

The SNP understands that. On Thursday, Nicola Sturgeon seemed to reiterate that her party would not support a Queen’s speech that committed a government to renewal, knowing it may be a question never asked. Coalition, requiring formal agreement on key policies and a veto on others, has been ruled out by both sides. The most likely relationship is on a policy-by-policy, vote-by-vote basis. Trident renewal will be a fiercely contested issue in the next parliament, whoever is in power. The Tory attack has shed no light, only a self-defeating heat.