Suddenly British politics has gone Hollywood. That's not a reference to pin-up boy Clegg but rather to the old William Goldman maxim about the film industry: "No one knows anything." Talk to those whose opinions ordinarily come armour-plated and they'll admit they're flailing.

Fifteen days out from a general election, such folk usually have a very clear sense of what will happen. Not this time.

You'd think that a rash of opinion polls with the Liberal Democrats in either first or second place would have all three parties working out the angles. "But we don't know the angles," admits one veteran of elections past. "There's no map." They feel like the crew of the Star Trek Enterprise, beamed down on a planet with an utterly alien landscape.

An immediate reaction is to hope they'll soon rub their eyes and see that it was all, if not a dream, then a passing fancy. Volatility is the word of the hour, expressing the hope that what has suddenly come up will just as suddenly go down – and normal service will resume.

Few expect the polls to return to the status quo ante, but they cling to the possibility that the Lib Dem boom will prove to be a boomlet and that the party will return to its rightful place behind the others. One cabinet minister told me yesterday that he simply refuses to believe Labour can come third in the popular vote. The Labour core was solid, he said, and the evidence on the doorstep told him support was "hardening". It will be all right on the night.

Optimistic types reckon the Lib Dem tide will rise just enough to lift Labour boats – perhaps by leaving them with the largest number of seats regardless of the popular vote – but not so far as to sink them. They avert their eyes from the polls that put Labour in the mid-20s and savour instead the sight of the Tories, once apparently cruising towards Downing Street, now hovering around the 30% mark. A senior Labour adviser confessed he has been "pretty delighted" to see the smile wiped off David Cameron's face.

For those wearing the rose-coloured glasses the very unpredictability of the election is itself a cause for cheer. What was a foregone conclusion – Labour defeat – is now up for grabs, and that counts as progress. Yes, there's frustration at Labour HQ that when it comes to media coverage, they are barely getting a look-in – "It's either Clegg or ash," moans one operative, lamenting that Monday's long-planned pitch on the economy hardly made a dent – but the larger reaction is nervous excitement: maybe we're not dead after all.

That has to be wrong-headed. For one thing, the battle now is about the mantle of change: Cameron had it, and last Thursday Clegg snatched it off him. But that competition excludes Gordon Brown. Whatever else the prime minister can offer – reassurance, experience – he can't be the champion of change, not after 13 years at or close to the pinnacle of power. The effect is to push Brown to the sidelines, the place he might occupy at tomorrow night's debate – watching as the two younger men duke it out.

Of course, there might be an electoral upside to that, the Tories and Lib Dems splitting the "change" vote and helping Labour grab more seats than both. This is the prospect exciting those party optimists. But it's surely a false hope. What kind of moral mandate to govern would Labour have if, for example, it came third in the popular vote?

That same cabinet minister gave a test-flight for the arguments Labour would probably run on 7 May in response, insisting that these were the rules the election was fought on, like all elections before it. And Cameron, defending first-past-the-post again yesterday, would hardly be able to object. Other Labour officials are saying that they could hardly be expected to invent a new constitution over the weekend of 8/9 May – and the markets would go nuts if no administration were in place by Monday morning. Brown would dutifully form a government, muttering apologetically that it's not his fault he won most seats.

But he would surely be drowned out by a furious chorus, enraged that a twice unelected politician should win bronze and yet ascend to the top of the podium. It would be unsustainable. And Labourites now heartened by the polls should look closely: they project the party winning a lower share than Michael Foot in 1983. And he at least came second.

Perhaps we should think bigger, towards the great progressive reunion of a centre-left that remained divided for most of the 20th century. Some lick their lips at such a prospect, but there are reasons to pause here too.

The condition of a Labour-Liberal alliance would be proportional representation. That's needed now for Britain, but it will all but ensure Brown's place as the last Labour prime minister to govern with his own majority. Even if that doesn't trouble independent-minded souls – and some among Labour's younger generation – it is a prospect that chills the likes of Brown or Jack Straw or Harriet Harman or Ed Balls. In other words, if the price of a deal with the Lib Dems is PR, it will come at a bloody internal cost. A civil war could be a matter of weeks away.

So much for the view from Labour. If Labour were quick to see the silver lining in the Clegg phenomenon, then the Tories were overwhelmed by the cloud. Proof came on Monday with Cameron's scrapping of a lavishly produced election broadcast, replacing it by a home-made effort shot in his garden – palpable proof the campaign was rattled.

Tory insiders say they are not working through the scenario of a second-place seats finish on 6 May. They also say the situation is too "volatile" to speculate (though a political discussion without speculation would be a short one). And they also have their optimists. One former party strategist says that even if the current polls are right it would not spell disaster. The Tories will be able to claim they topped the popular vote – and outpolled Michael Howard in 2005.

But that is to set the bar miserably low for a party that six months ago believed it was on the brink of power, even a 1997 transformation. There would surely be a bout of soul-searching as to how the Tories could have failed to defeat outright a government emerging from the biggest recession in memory, fighting an unpopular war and saddled with a leader much of the public can't stand.

As Labour knows only too well, a fourth consecutive defeat inflicts deep trauma on a political party. It forces it to contemplate the most searing change. Some will insist that Cameron's modernisation drive went too far, others not far enough. Many will demand his scalp. And how long could the Tories defend an electoral system that had cheated them of power?

In this alien landscape deep craters beckon for Labour and Tories. Only the Lib Dems, one imagines, would welcome it. Yet they look as baffled as their rivals, Clegg apparently overawed by his success, keen to dampen expectations. He too faces grave danger. The purity of opposition has kept his party together; might not the test of power – and whom to share it with – drive them apart?

It's possible that none of this will come to pass, that the bubble inflated last Thursday will burst. But as long as it endures there is the prospect that British politics is about to change for ever. Even by Hollywood standards, there's a lot riding on how this story ends.