Today's Tory intervention could have been focused on the question of whether a Labour government would renew Trident, sowing fear among voters about the possible influence of the pro-disarmament SNP. But rather than that policy issue, it is Michael Fallon's crude ad hominem attack on Ed Miliband that is dominating discussion. In today's Times, the Defence Secretary declared that Miliband "stabbed his own brother in the back to become Labour leader" and would "stab the United Kingdom in the back to become prime minister". The remarks were immediately identified by commentators as unworthy. "Embarrassing: Way too personal," tweeted Tim Montgomerie, the former editor of ConservativeHome. Later asked at the Q&A that followed his speech whether Miliband was a "decent" man, Fallon twice refused to say he was.

Swiftly made aware of the Defence Secretary's response, Miliband had an answer ready when asked about the issue at Labour's education manifesto launch. "Michael Fallon's a decent man," he said (thus paying his opponent the compliment he wouldn't), "but today he's demeaned himself and he's demeaned his office". By so personalising his attack on Miliband, Fallon handed the Labour leader a chance to be the statesman. As in the case of similar assaults on Miliband (most notably the Daily Mail's slurs against him and his father in 2013), it has backfired. "The Conservative Party can throw what they like .... but I've got used to it, I'm resilient", Miliband continued, adding that "decent Conservatives across our country will say 'come on we're better than this'. David Cameron should be ashamed." When later invited to endorse Fallon's "stab in the back" charge, the Prime Minister pointedly declined to do so, instead seeking to draw attention back to the issue of Trident. But the damage had been done. By refusing to say that Miliband was a "decent" man, Fallon merely allowed him to prove that he is (indeed, "decency" has long been one of the Labour leader's best-rated qualities among voters).

Labour MPs testify that Miliband's defeat of his brother in 2010 is often raised by voters on the doorstep - and rarely favourably. But it does not follow that the Conservative Party will profit from pursuing this line of attack. To claim that this act of "backstabbing" (also known as winning an election) proves that Miliband is prepared to relinquish Britain's nuclear weapons is to stretch credulity beyond repair (Labour today repeated that it is committed to full Trident renewal). Voters do not want or expect their politicians to resort to such tactics; they demand better.

The revived charge of fratricide also sits uneasily with the Tories' continual depiction of the Labour leader as "weak". Indeed, it was previously abandoned for fear of making him appear strong and decisive. That the Tories, less than a month before the election, are still unable to settle on a line of attack against Miliband is evidence of a campaign in trouble. As I write in my column this week, the Labour leader and his party have proved far more resilient than the Conservatives expected when they greeted his election with cheers. If today has shown that the Tories are prepared to plunge new depths in pursuit of victory it has also shown why they are unlikely to succeed.