Mere barter has replaced a devalued political currency in this General Election campaign.

That much is well-established. Instead of a competition over policies, we have haggling behind the scenes. Everyone knows it. Only Ed Miliband and David Cameron still try desperately, therefore comically, to pretend otherwise.

It's not their only pretence. Those two aside, the only people betting against a hung parliament now are daft types who always pick the longest shot, just for fun. The only ones who don't admit that deals, pacts, concordats - you can have your choice - will emerge are those paid to tell party leaders how jolly well they're doing.

For everyone else, there are refined words and phrases to enjoy: fragmented politics, legitimacy, stability, mandates. None of these means much beyond the simple truth that the old Westminster revolving door has been kicked in. It will not be repaired soon. Contrary to hysterical sorts, this does not mean the end of civilisation as we know it. Already, nevertheless, it is producing fascinating phenomena.

Return to Mr Miliband and Mr Cameron. Do they know how much the political universe has altered? If so, are they incapable of admitting the truth, terrified of the truth, or bereft of a coherent response? While they persist in their pantomime roles as leaders pursuing outright majorities, their parties are fading away, shrinking before our eyes.

You might think the claim absurd. One of the two will be prime minister after the dust has settled, after all. Labour and the Conservatives remain easily the best-supported parties in the United Kingdom. A few upstarts jostling for influence have not - you might argue - shifted the centre of gravity of political life. To listen to their stump speeches, that remains the view of Mr Cameron and Mr Miliband.

It deserves a question: so how did you get into this fix, scrapping it out for a handful of the last votes your parties did not lose long ago? In terms of shares of the vote, general elections were once carved up between Labour and Tories. Generation after generation, the line on the graph has drifted ever downwards. Where might it one day stop? And then what?

You could find many reasons for a decline that has seen Conservatives and Labour confined to (roughly) one-third of the electorate each. Disgust with Westminster - and the carve-up - has been potent. The Scottish National Party has torn up a lot of the venerated rules for first-past-the-post contests. Ukip has reminded Mr Cameron that the Conservative brand never stays "detoxified". A growing number now want more than token green politics. And the LibDems have been crucified for dishonesty.

If stability is the issue tantalising the hacks of Old London Town, meanwhile, the best place to start is with the UK itself. Much as Labour tries, this cannot be written off as merely "constitutional". The social and economic pressures on the UK edifice in all its versions grow more intense. You could draw them on a map, the isobars of lived experience. On that map you could scrawl crude tags: "Here be hedge fund managers; here lie food banks." Few other features would show a kingdom united.

The point would be that fewer and fewer voters believe in the ability or willingness of the old, big parties to recognise or deal with such facts. What Mr Miliband and Mr Cameron like to call answers are not believed. The old rhetoric of speaking (or governing) for Britain no longer applies. The rough compromise - "So let's give the other lot a turn" - is no longer accepted. The usual party concentration on the usual lists of swing seats might squeeze a few votes, but it will not turn back the rising tide.

Some functionaries will list all those "factors". Now and then - less so since Scotland's referendum - they will bewail apathy. What they will not do is confess failure. Defeat tomorrow will mean the end of leadership, sooner or later, for Mr Miliband or Mr Cameron. Then their careers will be assessed. In time they will probably write books to explain what it was all about. But whether either would want to discuss the long, inexorable decline of two old UK parties is an open question.

Mr Cameron might care to explain what became of a Tory revival in Scotland on his watch. These days, he barely bothers to pretend that such a thing is conceivable. Mr Miliband would have an even trickier task. What was his role, if any, while Labour in Scotland destroyed itself? Either phenomenon is historic, plainly enough, with consequences we have not heard the last of, but both are emblematic.

Labour crushed in Scotland? So where in the UK is Labour - hand on heart - safe for another generation? Scottish Tory MPs (if any) irrelevant to the government and future of that UK? So on whose behalf would any British Conservative leader seek to govern? And still the bigger questions: why did this happen, and where does it leave us? There is a loss of belief that goes beyond shifting allegiances. It raises the real question of legitimacy.

First-past-the-post was supposed to be Westminster's guarantee of stable government. "Stability" was always the answer given to anyone who questioned a system that patently failed to reflect the wishes of all voters, or to treat smaller parties fairly. That system is now an rumbling crock being towed to the scrapyard by the SNP. This is, with a special kind of irony, one of the very events the ancient fix was supposed to avert.

In their different ways, Mr Miliband and Mr Cameron are struggling to find ways to reach Downing Street by denying reality. Deals, coalitions, constitutional fixes, another spin on the abacus of seat calculations: something will turn up. What will no longer protect their careers is first-past-the-post and the familiar duopoly, even when ever-helpful TV reporting pretends that nothing has changed in Britain's politics.

To sort this out, the UK will have to reform its electoral system. To do that, it will have to cope with the Tories and Labour kicking and screaming for their ragged comfort blanket. If the UK wishes to hang together - and you never can tell - it will also have to accept that all the familiar excuses of "stable government" will have to go. A multi-party politics tends not to produce those beloved decisive results. It also - witness Israel - can allow tiny factions to have a lot of influence.

Better that, on a proportional basis, than two expiring dinosaurs still refusing to notice a change in the weather. After the votes are in, the UK will have a government that has inched towards a recognition of political diversity within these islands. Mr Cameron and Mr Miliband won't care for it much, irrespective of their chances to call on the Queen, but both should remind themselves of what has become of their parties in Scotland.

You needn't call it an omen. Omens have been visible for Westminster for years. But it is never a bad idea for a political species to evolve if it still has the chance.