I hope you're not looking for 20/20 foresight. If so, you may have come to the wrong place. About 10 minutes after the conclusion of the first leadership debate I wrote these words for publication the following day: "It was, the ITV continuity announcer said breathlessly, 'a first in British TV history' – but it contained little that history will want to preserve beyond today's papers."

In mitigation I wasn't the only person who felt that way at the time. Nick Clegg, it was later reported, shared that view – tugging off his lapel mic at the end of the broadcast with the conviction that he'd not made the most of his unprecedented opportunity to play in the Premier League. What neither of us had allowed for, though, was that the precise details of his performance on that night scarcely mattered. Turning up was enough in itself, that third podium on stage a novel impurity in the mix that suddenly crystallised a super-saturated solution of political disenchantment.

It wasn't so much that Clegg was dazzling – it was more that he had a light shone on him and did nothing to drive people away. Before the debate, a YouGov poll had respondents predicting a last-place finish for Clegg by a very substantial margin. His triumph was an object lesson in the value of Low Expectations – a virtue known to every spin doctor who's ever pitched his candidate as the underdog, but acquired here by Clegg without any need for spinning.

There's some mitigation, too, in recalling that that first debate wasn't quite the seismic game-changer that excitable journalists described in the following two days. True, it had been broadly felt that the election was Cameron's to lose. True, too, that the Lib Dems had occupied their traditional place in the polls – a poor third to the other parties. But the long prelude to the opening of the campaign was hardly a procession to a foregone conclusion.

In late February, a Sunday Times poll had even warned of the possibility of a small Labour majority. Other polls had suggested that, while that might be a touch farfetched, the current Government could scrape by in a hung parliament. And while all elections belong to the floating voters, in this one electoral indecision seemed to have taken on an extra depth. Shaken by expenses scandals and the financial crisis (which left Parliament looking like a chocolate fireguard), it was as if we couldn't even decide whether we wanted an election at all. The fate of the Conservatives' first campaign poster – David Cameron's airbrushed features alongside an invitingly blank space – suggested that the electorate was in the mood for a bit of creative vandalism. "Some of my best friends are poor" read one of the variations on the curiously glum official Tory slogan – "We Can't Go On Like This" – a bit of DIY satire that was later visited on Labour's official posters as well. (When the history of this election comes to be written there should be a small appendix devoted to Photoshop activism and Twitter rebuttal.)

What was missing, though – until the night of that first debate – was any sense that this might be an engrossing or involving election. We were bracing ourselves for coverage of the leader's wives' sartorial choices, for the routine business of daily photo-ops and manifesto launches, for an election run by the old rules of money and publicity. There might be occasions for mild ribaldry – as there was when Labour unveiled a manifesto that looked like a Maoist cornflake packet (or an Iranian pre-emptive nuclear strike on Hertfordshire). There might be bickering about the parties' respective slogans – as when Tony Blair dismissed "Time for Change" as the most vacuous soundbite of the campaign. (The Labour Party rapidly closed the vacuity gap with Gordon Brown's contentless boast "We are in the future business".) There might even be modest innovations in political rhetoric – as when the Conservative manifesto extended an invitation to every voter in the land to "Join the Government of Britain". But the mood was perfectly caught by one pre-debate poll, in which the expectation of a Cameron Prime Ministership was high but the enthusiasm for that outcome pointedly low.

Locally, at least, the First Debate changed all that. We have still to find out just how historic this "historic" moment is going to be. Should the Liberal Democrats falter this week and leave two-party politics broadly undisturbed, then 15 April will be consigned to the footnotes of political history, the stuff of trivia quizzes. If they don't, then that date will get a chapter of its own, if not an entire book. But in the couple of days that followed the debate it was unquestionably seen as a potentially significant turning point.

The "yellow surge" was how some journalists described the astonishing upward spike in the Liberal Democrat poll results – a disquieting image which called to mind the sudden rupturing of an abscess. Others frantically coined neologisms: it was Cleggmania, it was Cleggstasy, it was Cleggphoria (that last one was said to be Alastair Campbell's) – all of them assuming that it was the personality of the Lib Dem leader, rather than the acknowledgement of his mere existence, that had connected the political process to the mains. "Is Nick Clegg the British Obama?" asked one newspaper, absurdly. "Is Nick Clegg the new John Sergeant?" asked another commentator, more sensibly – suggesting that what was being registered here was a voter revolt against being told what to do by an insider elite. And if Clegg was the new John Sergeant, it was clear that he wasn't going to pull out on the grounds that it had been fun while it lasted but it had all gone a bit far.

Suddenly, both established parties had a new official opposition. "Vote Clegg Get Brown" said Cameron on 19 April, alarmed enough by the Lib Dem leaders' buoyancy to try and chain an anvil to his leg. Gordon Brown – who had spent most of the first debate saying "I agree with Nick" – suddenly realised that he didn't after all, and certainly not if there was any possibility that Nick might have the power to say something other than "Yes, Prime Minister" after the results had come in. The Labour Party inner circle began squabbling over their best strategy – loudly enough for the sound to reach the outer circle and the world at large. And by this point the polls were dancing all over the place like a faulty altimeter. Were Labour about to hit a hillside? Or was there still time to pull out of the dive? In newspaper offices there were two new imperatives: find someone to produce "Your pull-out guide to a hung parliament" (ours is on page 12) and get some dirt on Clegg.

On 22 April, the morning of the second debate, Clegg woke up to discover the downside of actually mattering: savagely hostile coverage in traditionally Tory-supporting papers, which criticised him for his expenses claims and remarks trawled from ancient newspaper articles. "Clegg in Nazi slur on UK" read one headline. "I've gone from being Churchill to a Nazi in a week", Clegg joked in response – and a Twitter stream tagged #nickcleggsfault seemed to confirm his instinct that the electorate would find the calculated attacks more risible than worrying.

The second debate confirmed that his competent performance in the first hadn't been a fluke. Both Brown and Cameron were more confident, a bit more at ease with the protocols of television debate. But Clegg was as well, encouraged, if anything, by the fact that he now merited a tailor-made smear campaign, rather than the charity shop slurs usually reserved for Lib Dem leaders.

The yellow on-screen "worm" that recorded the second-by-second reactions of a panel of representative voters continued to wriggle encouragingly upwards. Conservatives who had now grasped how serious an error it had been to let him in on equal terms muttered disconsolately about X Factor politics and game-show popularity contests. And it became clear that these three one-and-a-half-hour television broadcasts had pretty much sucked the air away from more traditional methods of connecting with the voters.

Both Conservatives and Labour had had problems with posters, the Tories with their open invitation to the graffiti artists of Great Britain, and Labour with an ill-conceived attempt to depict Cameron as a Thatcherite throwback, perching him on the bonnet of Gene Hunt's red Quattro, with the line "Don't let him take Britain back to the 1980s". Realising that they had everything to gain from an association between their candidate and a no-nonsense anti-hero who nicks villains, the Conservatives pounced on the miscalculation. "Fire Up The Quattro. It's Time For A Change" read their version, the following day. But that was a blip: this wasn't an election that was going to be fought or won on the billboards of Britain. That it could conceivably be lost by the wrong kind of photograph was, of course, still a possibility – one acknowledged when David Cameron turned up to his sister's wedding on 24 April in a business suit, avoiding the morning suit and topper worn by all the other guests. The one thing he didn't need now was a Lord Snooty headline. And if he wasn't considering a political marriage himself just yet, he'd understood that he had to make it clear that he wouldn't rule one out. The Tories cracked open the door to the possibility of coalition.

On the Andrew Marr Show, on the morning of Sunday 25 April, the only available bridegroom found himself being closely questioned about his intentions. "Here we get into the 'what-if' territory that I find very difficult," Clegg said, startling viewers who knew that he'd spent his entire career in "what-if" territory and should by now know its every rat-run and back alley. Was he keeping his options open? Or was he feeling the momentary panic of the learner driver who finds, after hours spent on the theory of acceleration and braking, that the real thing is alarmingly volatile? He seemed to rule out a coalition with Labour and then, startled by the ensuing jolt, hit the accelerator again to indicate that it was possible after all. The Labour Party, in the path of this erratically driven vehicle, couldn't decide which way to jump. Alan Johnson acknowledged that a coalition had to be considered. Ed Balls insisted that it wasn't "the British way of doing things".

There was a brief interval for more conventional campaigning. Cameron tried to seduce ecologically minded floating voters with some soundbites: "When you vote blue, you go green," he announced – a line that suggested that tactical voters wouldn't just have to hold their nose but take nausea pills, too. Those travelling with the Labour leader certainly needed something to calm their stomachs – as a rhapsodic hymn of praise to an audience of health workers reached its Hallmark card climax: "We have been in the presence of angels dressed in nurses' uniforms," he said. You half-expected a woman in uniform to intervene and lead him gently from the stage: "Yes Mr Brown... Angels... I'm sure... Now, I wonder if can you tell me the name of the current Prime Minister?"

The Tories, meanwhile, were holding a press conference at which the words "liberal" and "progressive" occurred so often that you wondered whether they were considering a name change – not Labservatives, the coinage with which the Lib-Dems had attacked the two-party hegemony, but Coniberal, perhaps. And to press their strategy home they pulled their planned Party Political Broadcast and replaced it with a pitch on behalf of the Hung Parliament Party, in which a spokesman solemnly pledged that "policies will be bickered over by secret committees" and the UK economy would be paralysed. "This is what a hung parliament looks like," the final title card read, over a picture of Gordon Brown grinning outside No 10. And then – startlingly, even deliciously for the crueller observers among us – the writers of The Thick of It took control of the Labour campaign, crafting its narrative line not as a conventional bid for office but as a master-class in political farce. On 27 April a cartoon character, Peppa Pig, withdrew from a planned Labour event; pigs are notoriously omnivorous creatures, employed in some countries as rambling sewage treatment plants, but this particular swine appeared to have turned its snout up at an association with Labour education policy.

The gaffe famine seemed at last to be over, though nobody could have had any idea yet just how lavish the feast was to be. On 27 April, David Cameron had discovered the risks of interacting with ordinary people when he found himself being berated for Tory policy on special needs education, by a man whose son suffered from spina bifida. The following day it was Gordon Brown's turn – in an encounter that is likely to remain an international benchmark of on-the-stump humiliation.

After a relatively innocuous encounter with a Rochdale Labour supporter, Gillian Duffy, Brown failed to turn off his microphone – and the discrepancy between his public and private face was broadcast to the world. That there was a discrepancy was hardly a scandal in itself, but the impression of a man panicked by the mildest kind of contradiction, and looking immediately for someone to blame, was too close a fit to criticisms of his psychology to be anything but devastating. Brown compounded the disaster when, listening to a recording of his remarks during a scheduled radio interview, he let his head slump forward in despair.

After a 40-minute private grovel in Mrs Duffy's front room, the smile had been stapled back into place, but nobody could any longer be in doubt about what it concealed. A few days later, as Labour exhaustedly tried to re-energise its campaign (and effectively de-Brown it by acknowledging the existence of his colleagues), Lord Mandelson found his speech interrupted by the comedy sound-effect of a nearby car-crash, provoked, it later turned out, by the fact that the occupants of one of the vehicles involved had been shouting abuse at the politicians. Only if the next photo-op had been in a goat-breeding centre would it have been possible to come up with a more apt metaphorical match for the shambolic state of Labour's campaign.

That Brown turned up at all to the third debate was a kind of heroism, but it hardly mattered that this was his specialist subject: the economy. Nothing he did could budge the approval ratings upwards. Cameron, as if energised by just how terrible the week had been for Labour, came out stronger and Clegg's central pitch began to wear a little thin. "There they go again with the political point-scoring," he said, a line only a three-year-old wouldn't have categorised as political point-scoring. All three men underlined how important it was to be honest with the electorate about the scale of deficit; not one of them was – staying true to a campaign distinguished by the rarity of serious debate on political policy.

We entered the final days of the election with Cameron issuing "A Contract Between the Conservative Party And You" and the Labour Party changing tack again to campaign under a new slogan: "Fighting For Your Future". (Nobody seemed to have noticed the typo which had attached a stray "Y" to the front of the pronoun.) And – wonderfully – we learnt that Nick Clegg's literary hero is Samuel Beckett. Those of far greater political percipience than me have suggested that this revelation is unlikely to lead to any significant shift in the voting intentions in critical Northern marginals, but we can surely treasure the information even so.

What more fitting choice could there be for a Lib-Dem leader than a writer whose masterpiece concerns men marooned in a state of perpetually disappointed expectation? Perhaps this time round Godot really will come and this will be an election in a generation. Or perhaps it'll just be Endgame instead. Either way, he has played a part in a campaign which had far more surprises and plot twists and cliff-hanger tension than even the most optimistic could have predicted three weeks ago.