For yet another day, the Conservatives are pushing the line that a minority Labour government would be held to ransom by the SNP. In his first intervention of the election campaign, John Major will warn in a speech that the nationalists would hold sway over economic policy. "Labour would be in hock to a party that - slowly but surely - will push them ever further to the left," he will say. "And who would pay the price for this? We all would. We would all pay for the SNP's ransom in our daily lives - through higher taxes, fewer jobs, and more and more debt.” The Tories regard this attack as a powerful means of winning over Ukip defectors and Lib Dems in southern battleground seats.

In recent weeks they have warned that a Labour government dependent on the SNP for support would be forced to abandon Trident and captiulate on deficit reduction. But this rhetoric masks what would be a very different reality. Firstly, as I wrote last week, the SNP has already dramatically reduced its bargaining power by vowing not to prop up any Conservative-led government. In the case of Trident, as Stephen noted yesterday, those MPs in favour of renewal (most of Labour, the Tories and the Lib Dems) will vastly outweigh those opposed. As long as the Tories are prepared to walk through the division lobbies with Miliband, there is no chance of the SNP blocking defence spending (as its deputy leader Stewart Hosie grandiosely suggested it would).

The nationalists' hand is little stronger in the case of the Budget. We are told by Major and others that the SNP could flood Labour's programme with tax rises and spending increases. But as Colin Talbot, professor of government at Manchester University, notes in an essential post: "In the Westminster parliament only the government can propose taxation or spending measures. These can be defeated, or amended, but only by cutting spending or lowering or removing taxes – not by increasing either." There is no parliamentary means by which the SNP could force a Labour government to spend £140bn more on public services (as proposed in its manifesto). It is this that explains the confidence of Ed Balls in asserting that he would not "negotiate" with the party over the Budget and that its measures would be entirely determined by himself and Ed Milliband.

The SNP could, in theory, combine forces with the Tories to vote down a Labour Budget (without fear of triggering a second election owing to the Fixed-term Parliaments Act). But would it really be prepared to obstruct measures endorsed in its manifesto such as the reintroduction of the 50p tax rate, the repeal of the bedroom tax, the abolition of non-dom status and a bankers' bonus tax? The sight of SNP MPs voting alongside Conservatives to prevent such progressive policies would be a gift to Scottish Labour.

For these reasons, whatever the political merits of the Conservatives' attack (and there are some Tories who fear it is crowding out their core message), it deserves to be treated with far greater policy scepticism than at present. The SNP talk a good game but they would struggle to play one.