Why won’t Ed Miliband commit to a deal with the SNP in advance? The explanation is incredibly simple. Here are the last three Scottish polls – Survation for the Daily Record, TNS Global, and Ipsos MORI. As expected they both show a monster LAB>SNP swing. But the interesting bit is this: there are a lot of undecided voters.

TNS, for example, shows 29% of their sample as Undecided. That’s twice as many as all the other parties other than Labour or the SNP. That’s as many Don’t Knows as there were Scottish Nationalists. That’s more than the gap between the SNP and Labour. The biggest groups of DKs are the young (34% of 18-34s, 38% of 18-24s). Socioeconomic groups C1 and C2 and women are the other likely DKs, but the difference from the national result is much smaller.

Survation formulated its questions rather differently. As a result, you might think TNS got a dodgy sample, as they only saw 11.5% Undecided. However, they also asked those respondents who picked a party in the voting intention question if they might change their minds between now and polling day – i.e. if they might actually be undecided. They found 19.2% were still, in this sense, undecided.

You can’t add the two percentages, because the second only includes those who picked a party. But we have the full tables, so we can tot up the 163 undecided here and the 111 earlier and divide by the weighted n=968, and conclude that 28.3% of Survation’s respondents are undecided.

Ipsos MORI, on the same basis, gets 26% undecided/DK. Pretty close!

If you’re Scottish, you are much more likely to prefer a Labour government to a Conservative one. TNS makes the split 35% to 16%. Therefore, if you’re undecided you are most likely wavering between a Labour-first route to this or a Labour-SNP route. The most immediate reason to prefer Labour is obviously that voting SNP denies Labour seats it needs to form a government and therefore risks a Tory-[something] coalition.

If Ed Miliband was to pre-commit to an alliance with the SNP, this possibility would vanish and with it, any chance to save seats in Scotland. Also, you know if he thought there was any advantage in stabbing Scottish Labour in the neck he’d already have done it.

With almost a third of the voters still in play, there is absolutely no point in giving up. This ain’t the movies, so it’s not as if all 29% of them will break for Labour, but there is a possibility that the SNP will disappoint significantly on the night. Rather like they did in the referendum, in fact. Also, Survation provides the interesting detail that the biggest group of people who might change their minds, out of those who named a party, are ex-Lib Dems, or in other words, the group of people in the UK most likely to support Ed Miliband.

That’s why Miliband won’t pre-commit to a deal with the SNP.