Iain Watson: What sort of deal would Miliband do?

With polls suggesting a hung parliament - and potentially heavy Labour losses to the SNP - it's inevitable that when Ed Miliband visits Scotland, he will face questions about any deal he might strike with Nicola Sturgeon's party to keep the Conservatives out of power at Westminster.

He has for some time ruled out a formal coalition. So has she.

So speculation has moved on to whether they could reach an informal arrangement - "confidence and supply" - where the SNP would guarantee a Labour budget in return for concessions.

For a time, relatively measured language was used towards the SNP as Labour tried to woo their former supporters who had voted "yes" in the referendum.

The rhetoric is now more robust as the polls refuse to budge.

But the underlying thinking is much the same.

'Runner-up' as PM?

Because the SNP has promised the Scottish people they won't help David Cameron into Downing Street, if Labour have the most seats but fall short of a majority they say they would simply challenge the nationalists to back a legislative programme - their Queen's Speech.

While they would talk to SNP and indeed DUP MPs, as Angela Eagle, shadow leader of the house told the BBC, no deal - formal or informal - would be forthcoming.

Instead that Queen's Speech would be sure to include policies which those other parties would vote for - such as the abolition of the under-occupancy penalty or bedroom tax - to ensure it passed and to minimise the opportunity for mischief.

If the SNP took down a Labour government promising coincidentally to implement some of their policies, their chances of doing well at the 2016 Scottish Parliament elections would be diminished.

Further down the line, the SNP could vote against Trident - but enough Conservatives would back renewal for a Labour minority government to avoid defeat.

The trickier question is what happens if the Conservatives have more seats than Labour. Could Ed Miliband still become prime minister with tacit SNP support?

Would he accept a "runner-up" could be PM? That's a question he hasn't yet answered.

Constitutionally it would be possible, but politically it could damage Labour's future prospects in England.