Ed Miliband, Nick Clegg and David Cameron
The three party leaders can't guarantee delivery of their 'vow'Reuters

Why would any Scottish voter believe the unprecedented "vow" delivered by all three Westminster party leaders in their final, panic-stricken desperation to save their Union, and their necks?

Within hours of the extraordinary declaration on the front page of the Scottish Daily Record paper, it was already being claimed the vow was not worth the paper it was written on.

There are numerous reasons for questioning the vague promises of more power to Holyrood, but the most glaringly obvious is the simple fact that the three leaders are in no position to promise they can deliver.

Leaving aside the fact that the vow is vague and lacking in detail, the fact is, significant numbers of both Labour and Tory MPs are deeply unhappy at the way Scotland appears to be being offered preferential constitutional rights and powers over other nations and regions.

Many have also long been opposed to the continuation of the existing funding deal, the Barnett formula, which they believe is weighted in favour of Scotland and which the three leaders have now promised to continue for ever, just as the demands for its reform were gaining ground.

Even the man who invented it before the 1979 devolution debate, Labour's Joel Barnett, now a peer, believes it has outlived its usefulness.

So the idea that the next government will simply put this hugely significant new constitutional deal before MPs and it will sail through both houses of parliament without a murmur is pure fantasy.

Even the comparatively simple reform of the House of Lords, launched by Tony Blair, turned into a farce and, ultimately, ended with a halfway house no one wanted or voted for. All parties are committed to completing that constitutional change, but no one expects that to happen soon, or at all.

Conservative leader of the Commons, William Hague, put the whole thing in context when standing-in for the prime minister in the Commons last week.

He faced anger from two senior backbenchers, Christopher Chope and John Redwood, over the constitutional impact of the proposed "devo max" for Scotland.

Redwood represented the tip of one looming iceberg which will threaten the government after the independence referendum, whatever the outcome, when he pleaded: "Who speaks for England?"

Chope wanted to know when the government had changed its policy on devo max.

Hague responded: "The statements by the party leaders made on this in the last few days are statements by party leaders in a campaign, not a statement of government policy today, but a statement of commitment from the three main political parties."

He added that the promises were: "Akin to statements by party leaders in a general election campaign of what they intend to do afterwards."

That comment, on its own, should be enough to give voters pause for thought. Clearly, party leaders cannot guarantee that their election promises are then delivered by whoever forms the following government – even when they all agree.

House of Commons
Constitution will dominate parliament businessReuters

That is one of the reasons for the disillusion with the current system. No one has forgotten Nick Clegg's tuition fees vow before the last election, also signed, sealed and not delivered.

But when those promises are made in such historic circumstances they deserve to be tested to destruction. And the truth is they cannot be pledges or vows, they can only be hopes; hopes that whoever leads the next government will be in a position to win over enough doubters in parliament to get it on the statute books.

In any case, and again irrespective of the outcome in Scotland, there are likely to be powerful demands for a much wider constitutional debate to tackle all the consequences of the referendum decision and the impact it will have across the UK.

David Cameron has, so far, brushed aside demands for things like an English parliament or English executive after the referendum. But these demands will not go away.

So the remaining months before the general election will be dominated by the greater constitutional wrangling. Forget the economy and the cost of living crisis.

And whoever forms the next government, they will get bogged down in a constitutional quagmire of their own making of which Scotland will only be a part.