Can you win a landslide, and still be powerless? The SNP might be about to find out. It now looks as if the party is nailed on to win at least 50 seats in Scotland and it’s not unreasonable to suppose that they might win every seat on the Scottish mainland.

But what can they negotiate with, exactly? They’ve said before, and reiterated in their manifesto that they will not work with the Conservatives in any circumstances. Their leverage over Ed Miliband, my colleague George has blogged before, is vanishingly small. Stewart Hosie, the deputy leader, gave the Conservatives some good headlines with his threat to defund Trident through the budget or shutdown the British government thereafter, but neither weapon looks particularly sharp to me. Whatever the outcome of the next election there will be a large pro-Trident majority whatever the SNP, Plaid Cymru, the Greens or the left flank of the parliamentary Labour party do. It is possible that the SNP could vote down Labour budgets thereafter – the Fixed Term Parliaments Act means this wouldn’t necessarily result in an election – but will the Nationalists really inflict a shutdown of government services on their own voters? It doesn’t seem likely and would be electorally lethal.

So the end result is a position of electoral strength and political weakness at Westminister, and as a result the SNP’s manifesto will leave the reader with a sense of déjà vu. A commitment to restore the 50p rate of income tax, a raid on bankers’ bonuses, the end to the married couples’ tax allowance, the abolition of non-dom status and further taxes on expensive homes: these are all Labour policies.

Far from offering a radical left alternative, it feels as if it is the SNP being pulled leftward by Ed Miliband. These are, for the most part, policies that have long been championed by the Labour leader but opposed by the Nationalists. There are areas where the Labour leader has been outbid; a minimum wage of £8.70 an hour as opposed to £8. But on housing, the SNP fall badly short, promising half (100,000 a year) the new homes that Labour offer (200,000 a year). Tax breaks for businesses paying living wage has been Labour policy for several years now, and is in fact already being implemented by Labour-controlled local authorities in England.

The trouble for the SNP, is, having ruled out any alliance with one party, they will have a hellish time getting concessions out of the other, particularly as Ed Miliband will be loath to be seen to concede anything to the Nationalists. (And the more seats Labour lose in Scotland this time around, the less they will have to protect over the next five years.) It may be that, far from providing a stronger voice for Scotland at Westminster , the SNP find themselves largely ineffective in the Commons.

Although that, of course, isn’t necessarily the worst thing in the world as far as their big argument is concerned.