The tax bombshells are going off already.

Black holes hitherto unknown to science are appearing all over the place. There are only five minutes left to save the NHS and the immigrant hordes are at the gates. For a 21st century general election, it's all very quaint.

Two facts are in collision. One is obvious: the contest that began on Monday has no precedent in recent decades. First-past-the-post, that old bastion, has not surrendered to the popular will so completely since the Labour Party was busy being born. Received wisdom has been abolished. All bets are void.

Yet, for light relief, there's the other fact: David Cameron, Ed Miliband and Nick Clegg are campaigning as though nothing whatever has changed. No sooner had the Tory leader returned from reminding the Queen to keep an eye on Monday's news than he was standing in the middle of Downing Street, keeping the big front door in shot and denouncing Mr Miliband.

Thrice was the Labour man named and, if that's your taste, shamed. Mr Cameron hawked his unbeatable bargains -growth, jobs, an improved NHS, tax cuts, national security - and did some negative advertising on Mr Miliband's shoddy goods. Vote for Ed, said Dave, and there would be "chaos" (the new favourite Tory word) and £3,000 of tax increases for every working family. "Debt will rise and jobs will be lost as a result," said Mr Happy.

The £3,000 bombshell soon turned out to be a dud, but that was of no concern to Mr Cameron. He had mentioned the words "chaos" and "Miliband": his campaign was launched. Meanwhile, the Labour leader accompanied his "business manifesto" with an attack on Conservative plans for a referendum on European Union membership. For a wonder, he also managed to mention chaos.

When he wasn't amusing the press with a visit to a Solihull hedgehog sanctuary, Mr Clegg was struggling to squeeze into a costume that last fitted five years ago. Labour would veer to the lethal left, he told anyone listening, and the Tories would lurch to the risky right. If the country wanted a sensible chap to anchor government to the sensible centre ground, voters need look no further. Nick stood ready for someone to agree with him.

It was all entirely, tediously familiar. It was all reported dutifully. It was all acted out as though a tattered soap opera script from 20 years back had been mistaken for the latest episode. That's not fanciful. In 1992, the Tories were prophesying a Labour tax "double whammy" and warning that those "floodgates" would be opened to immigrants. Labour were taking every opportunity to talk about NHS waiting lists and private medicine.

It would be wrong to say that the old party machines are entirely unchanged. In 1992, a red £5 souvenir mug bearing the words "Controls on Immigration - I'm Voting Labour 7 May" would have been regarded by the party as a sick joke. In 1992, even with opinion polls predicting a tight election, the Tories were still guardians of a Thatcherite legacy with 13 years in government behind them, not a faction trying to win outright for the first time since John Major managed that surprise.

In 2015, nevertheless, they go at it as though elections in the United Kingdom still abide by the old rules: two big parties, two leaders, and the fiction of profound ideological differences. Everyone paying the slightest attention knows this is nonsense. The electoral landscape has altered out of all recognition since last autumn, far less since 2010. In reality, Mr Cameron has a lot more to worry about than just Mr Miliband, and vice versa. Mr Clegg's worries are beyond counting.

You can take the issues in no particular order. One is symptomatic: each of these three men is unpopular. A poll at the weekend had the Tory leader's "net personal rating" in positive territory for the first time in years by a statistically insignificant one per cent. Mr Miliband stood at minus 21; Mr Clegg at an abysmal minus 40. Since parties promote themselves in terms of individual leadership, this speaks volumes.

But then, these parties are themselves damaged goods. In 1992, the Tories, Labour and the Liberal Democrats ended election day with a 94.1 per cent collective share of the vote. Yesterday morning, the BBC's Poll of Polls put their joint share at 76 per cent. Mr Clegg's party, with just eight per cent, is struggling desperately to salvage what it can from impending carnage; the other two are locked, still, on 34 per cent each. There is, if you like, nothing to choose between them.

This has been going on, without much comment, for years. In 2010, these big parties had 88 per cent of the vote; in 2005, 89.6 per cent; in 2001, 90.7 per cent. Even if you take account of the LibDem collapse, the Blairite surge, or a general and growing disgust with Westminster, the familiar electoral facade has been crumbling for long enough. Yet still Messrs Cameron, Miliband and Clegg campaign as if only three riders matter in the 2015 race.

They have concluded, no doubt, that they have no other choice. What's striking, though, is the extent to which Labour and the Tories have given themselves over almost entirely to negative campaigning, to consolidating the vote they think they should have by scaring their "own" voters back into the fold. Throw Ukip at the Conservatives and Mr Cameron's party panics. Present Labour with an insurgent SNP and only the scale of the rout remains in question.

Suddenly it becomes obvious that the Westminster edifice was never as robust as the Lab-Con-Lib trio needed to pretend. In this, Labour's dismal tactics in Scotland are emblematic. In the space of just six months all the old cliches of heartlands and fiefdoms have been wiped away. All the fine referendum rhetoric extolling Scottish voters and their place in the Union no longer looks so clever.

On their showing thus far, Mr Cameron and Mr Miliband seem to think that if they ignore reality it will go away. Labour needs only two or three per cent of a lead to begin thinking about majority government. Forget the SNP: had his party avoided alienating even half of the five per cent who say they mean to vote Green, Mr Miliband could be scraping together a victory. But that, like any dream Mr Cameron has of clawing back support from Ukip, is a forlorn hope now.

The decline of those once-big parties and the advent of balanced parliaments has given voters a new understanding of what can be done in politics. It needn't be the same old shouting match between two men in suits with their threats and promises. This year's pair are fettered. Neither can govern unaided.

The SNP phenomenon, to take the most remarkable example, has a breathtaking strategy at its heart. It looms over Mr Cameron and Mr Miliband as they take their weary rep theatre back on the road: keep the Tories out of office and keep Labour honest. Within Westminster's known universe, that isn't supposed to be possible. The numbers say otherwise. And who's to blame for that?