Nobody ever votes for a coalition in the UK. Politicians sometimes pretend that people do. Listen to Nick Clegg on the subject of 2010: you'd think the Conservatives and Liberals put their coalition agreement to the public and it won a majority. But of course really people voted for their preferred parties and programmes and nobody managed to win. The coalition was an "accident" for the voters; a "deal" for the political class. That's the only way they can work in a First Past the Post system. To pretend otherwise is to deceive the voters.
And there's a degree of deception going on in Scotland, where some voters are being persuaded to believe that they are indeed voting for a coalition (or at least for a "progressive alliance"). The story goes like this: if people vote for the SNP they are not helping David Cameron or the Conservatives, they are voting for a bloc of left-of-centre MPs; for MPs who will back Miliband (while pushing him to the left on some issues): a win-win situation. Scottish voters can have it all: get rid of the Tories, push Labour leftwards and put Scotland first. What could be better than that?
Except, of course, it is an illusion. Our political system does not work like that. Labour seats going to the SNP does not mean they stay in a "progressive alliance"; it means that the Tories could be more likely to form the next government, and certainly are closer to being able to claim a superior mandate.
If Labour had a good year in Scotland, a slim majority would still be very much on the cards. That's the only sure way to get a Labour government. It doesn't look like it's going to be a good year in Scotland for them and every seat they lose in Scotland will erode Miliband's chances of forming a government. Why?
There are a few reasons:
1) One big question on election night is: which party came first? The fewer seats Labour win in Scotland, the more likely the Tories can claim that first "victory".
Of course, Clegg is extremely disingenuous when he declares that the party that comes second in an election should not be able to form a government. Liberal Democrat policy now appears to be that they should always be in government even if they come fourth in the election (in terms of seats) and fifth or sixth in terms of share of the vote. Also, the Liberal Democrats have always favoured electoral reform to a system where there is no logic to the biggest party having the first attempt to form a government: it's all about blocs of support. But that isn't the system we have in the UK. There is no constitutional necessity for the biggest party to lead the next government, but that does not mean that there won't be enormous pressure for that to be the case. There are no rules about what happens on May 8th if no party has a clear majority, but what precedent we have is that David Cameron should remain Prime Minister until a government can be formed. That puts the ball firmly in his court.
2) Scottish voters voting SNP have no control over how other people vote.
The logic behind the idea that Scottish voters voting SNP can only help a "progressive alliance" and not help the Tories can only even begin to stand up to scrutiny if we assume all the polls are correct and there is no chance of the Tories getting a majority or being able to muster a majority with Lib Dem and/or DUP and/or UKIP support. But of course that's not guaranteed! Okay, if the Tories manage to find a majority, then Labour holding onto more of their Scottish seats wouldn't prevent that (though it could hand them a bigger lead) but the chances of the Conservatives putting something together? Absolutely every Labour seat lost in Scotland helps make that a reality, because a big Labour lead could put it beyond question.
Imagine if Labour and the Conservatives both had 295 seats. They would each require the support (formal or otherwise) of another 30 MPs to win a confidence vote. Labour might be able to reply on 40 or so SNP MPs, 1 Green and a few Plaid Cymru in that scenario and feel pretty confident of leading a minority government. But it would only need the tiniest of adjustments - possibly many miles from Scotland - to foresee a situation where the Conservatives were able to form a formal coalition with 25 Lib Dems, 5 DUP and a UKIP: then they would have a majority and be able to form a government, even if Labour could have got bigger numbers, if they could have got the Lib Dems on side. Now the SNP could have reason to criticise other UK parties here. Why have Labour and the Lib Dems both ruled out any sort of deal with the SNP? That isn't their fault! And I have some sympathy with that view. Labour has effectively been pushed into that position (and the SNP have seemed to relish making it awkward for Labour, talking up their position of strength in any post-election negotiations). But there was really no pressure on Nick Clegg to rule out working with the SNP in all circumstances. Indeed his declaration on the SNP and on a party coming second forming the government is really further evidence that Clegg's Lib Dems are now part of a bloc with the Conservatives, however unlikely that seemed to some in 2010. It is astonishing that the Lib Dems have ruled out working with the SNP, but not the DUP. However, if Clegg loses his seat, the Lib Dem position may yet become more fluid.
3) There's an assumption that either SNP or Labour will always win the seats
Of course most of the seats in question will either be won by Labour or the SNP, but let us not imagine that it's impossible for some seats to see a split in the centre-left vote allowing other parties to succeed. I can't see a Tory or UKIP seat in Scotland (but there are areas that routinely voted Tory in the past and may well dislike the SNP's apparent left turn) but even a Lib Dem could slip through. The point is that ours is a system where votes cast can have unintended consequences. While an SNP vote in Scotland might be different from a Green vote in a key English marginal in terms of the laws of unintended consequences, it is still undoubtedly a gamble.
Of course, Labour have got some things wrong which has produced this situation. They fought the referendum battle in the wrong way and should not have worked so closely with Tories and other unionists and instead forged a separate, Labour, progressive case for a "No" vote. They have used poor rhetoric on the economy: actually the SNP's tax and spend policies are no more "anti-austerity" than Labour's, but the insistence on a rhetoric of "fiscal responsibility" to spike a few guns in the Tory press has backfired in Scotland. And let's face it the guns refuse to be spiked. It doesn't matter how much Miliband says his pledges require no more borrowing and he isn't going to do a deal with the SNP, the front page of the Tory press is still "Miliband and Salmond" and "more borrowing, more debt". I prefer the SNP policy on Trident to the Labour policy, as do most Labour parliamentary candidates. With a few days to go to the election, there's not a lot Labour can do about any of that. We can't rewrite the past.
But the Scottish voters have to look at the big picture. On May 8th we could have a government that will scrap the bedroom tax, abolish zero hours contracts, repeal the NHS Reform Act, cap rents and cut many billions less than a government that would make savage, eye-watering cuts to our public spending, with many billions more from welfare. It is not a choice between blue and red Tories; they are not "all the same"; it is a hugely important election for the whole of the UK and people in Scotland have the power to ensure a Miliband government and not a Cameron one. Other governments are not available.