We can all agree at least on two priorities: we want to see a Labour government after May, and we want to hold the Union together. So why are so many Labour members agitating to throw both those ideals away?

Look at the polls. The SNP are projected to win 56 seats in Scotland, becoming Britain’s third largest party. Plus, the Tories are projected to win around 20 more seats than Labour. Unless something drastic happens, there is a good possibility the Tories will end up as the largest party in May. Let’s start with that assumption first and go through the scenarios.

1) One possibility is that the Tories agree on another coalition with the Lib Dems and end up in government again. Disaster.

2) It’s also possible that Labour agrees on a coalition deal with the Lib Dems instead. But they are unlikely to get a majority in Parliament, and hence the legitimacy of a deal between the second and fourth largest parties would be questioned.

3) Labour agrees on a confidence-and-supply agreement with the SNP. But that wouldn’t stop the Conservatives (who are hungry for power) to strike a deal with the Lib Dems anyway and reach the finishing line. Their coalition would have more legitimacy than Labour’s deal. In fact, if Labour just relies on C&S, it opens up an opportunity for the Tories to strike another coalition deal and undercut Labour’s position.

4) Which brings us to the only option that guarantees Labour gets into power: a coalition deal, either with the SNP or with the Lib Dems, and the other signing a confidence and supply agreement. Which one would be more preferable? A coalition deal with the SNP would have more legitimacy, and it would undercut Tory attempts to offer key concessions to the Lib Dems and form a rival bloc.

But there’s another reason why a deal with the SNP is preferable: keeping Scotland in the Union.

This may sound counter-intuitive but consider this: a significant proportion of Scots now believe that Labour cannot represent their interests. They’re not even going to change their minds at the expense of a Tory government. Plus, the SNP are effectively using Tory rule through Westminster as a key argument for independence. Another Tory coalition would end the Union.

The only way Labour can make the argument for the Union now is by showing Scots it can serve their interests through Westminster, in a deal with the SNP. I suspect many Labour tribalists would balk at the idea, but there aren’t many options available to us.

Ruling out a deal with the SNP won’t attract Labour many new votes – most Scots seem to be tired of tactical voting, and are likely to just hold out for the Union to collapse naturally.

But by ruling out a coalition we not only make a Labour government less likely, but also end up being responsible for ending the Union. A coalition with the SNP must be on the table; Miliband has to consider what is good for the country not just the Labour party.