The election depends on LibDem and SNP unity of purpose, writes Robert Peston

The unlikely alliance of the Jo Swinson's Lib Dems and the SNP be key to sparking a general election. Credit: PA

It is a rum state of affairs when it is the two smaller parties, the Scottish National Party and the Liberal Democrats, that will decide whether there is a general election in December and not the official opposition, the Labour Party.

Because as I've been saying for days, it is the LibDems and SNP who've been making the running to find a way around the Fixed Term Parliament Act and force an election by simple majority.

By contrast the Labour Party has said it is in favour of an election, but not till after the risk of a no-deal Brexit has been eliminated, and without defining what would need to happen for that risk to be permanently eliminated (and as it happens, definitionally the risk of a no-deal Brexit can only be totally eliminated if the UK revokes Article 50 and stays in the EU - which is not Labour's policy).

It is also pretty rum of course that the government, led by Boris Johnson, is literally desperate for an election, because it can't govern in general and fears that even its cherished Brexit deal would be corrupted beyond recognition if it gave MPs as long as they want to scrutinise and amend it.

So this morning the SNP and the LibDems hold the fate of the nation in their hands, as they decide whether to have the courage of their convictions and force an election in December.

If Boris Johnson shifts nearer to their preferred election date of 9 December, and drops his insistence on 12 December, it may happen.

Boris Johnson has called on MPs to back a general election. Credit: PA

This matters to the LibDems in particular, who still bridle at being seen as Tory poodles in the coalition government of 2010-15.

As it happens, I am sure Johnson will not die in a ditch for 12 December. He will move.

But there is one other necessary condition for the election to happen: the LibDems and SNP have to continue to show a unified, united front.

Neither party can take the political risk ALONE of being seen to give Johnson what he says he wants.

Together they are able to make the credible argument that they should honour the letter they wrote to Donald Tusk, the EU's president, arguing for a three-month Brexit delay for the specified purpose of holding an election that they would support as as a route through the Brexit impasse.

Corbyn's Labour have so far refused to back a general election. Credit: PA

To run away frightened today would reinforce the completely reasonable and growing view in EU capitals that the British parliament is incapable of any action that could end the Brexit uncertainty. It would therefore increase the risk of EU leaders pulling the plug at the end of January, of a no deal Brexit then (though maybe too it would therefore throw Johnson a lifeline to get MPs' agreement before Christmas for his Brexit deal - which SNP and LibDems purport to hate).

SNP and LibDem MPs are meeting and deliberating this morning - and talking with Labour - to decide how to vote.

They stand together, or the election falls.