Since the election campaign began, David Cameron has been accused of lacking passion and of being "too posh to push". His manner on today's Andrew Marr show seemed like a conscious attempt to rebut this charge. "I'm angry and animated!" he declared at one point, lest anyone fail to notice. At moments, he rather too closely resembled an under-pressure chief executive or football manager facing the sack (repeatedly interrupting his interlocutor). But Cameron clearly believes that raising the rhetorical stakes is his best means of retaining power.

The opening of the interview saw him dramatically dial up the SNP attack, warning of the "frightening prospect" of a party that "wouldn't care about what happenened in the rest of the country" holding sway over a Labour government. In an attempt to make the danger less abstract, he suggested that an administration reliant on nationalist support would be forced to cancel infrastructure projects in England, referring to "People thinking in their own constituencies 'Is that bypass going to be built? Will my hospital get the money it needs?'"

But while excoriating Miliband for refusing to rule out a loose arrangement with the SNP (though the Labour leader is more likely, as I wrote on Friday, to simply call their bluff), Cameron took an equally ambiguous stance towards Ukip. Asked to rule out a deal with Nigel Farage, he merely replied: "We're not planning to do deals with anybody". Since polls show that voters are more concerned by Ukip holding influence in a hung parliament than the SNP (42 per cent against 27 per cent in a recent MORI poll), this is a weakness Labour should repeatedly exploit.

With Cameron currently on course to lose office, the Tories have resolved that their best hope of persuading wavering voters is to repeatedly play the SNP card - in an ever more apocalyptic manner. In particular, they hope that this will win over two key groups: Ukip defectors and southern Lib Dems. Whether or not the fear factor works, it is a disreputable campaign that only further undermines the long-term future of the Union. (As Marr observed at one point, Cameron sounded like an "English nationalist".) The Tories may yet retain power but they have already lost honour.