Live television has frequently been more dramatic. The moon landing. OJ Simpson. Diana's funeral. Have I Got News For You, scheduled head-to-head by the BBC, probably posted record ratings. As a broadcasting milestone, however, the party leaders' election debate was up there with the Coronation and the Speaker's first televised "Order, Order" from the House of Commons. When, just after half past eight, Alastair Stewart announced "Tonight, history in the making" another defence of the old political order crumbled, never to be rebuilt.

The early exchanges were full of tension. Nick Clegg will have savoured the chance to go first. Gordon Brown soon showed a command of orderly detail. David Cameron got his apology for the expenses scandal in first and was best at empathy. Gerard Oliver's opening question on immigration was hardly an easy place to start. But British elections are in a new place now. Better get used to it. Televised debates are here to stay.

Three questions matter in judging tonight's event. The first, inevitably, is how each of the leaders did – who won and lost, if you must. The second is whether the debate was good or bad for election turnout on May 6. The third is whether the format can be improved, either in the two remaining debates, which seems unlikely, or, more importantly, at future elections.

Clegg, treated fairly by the system for once and not barracked by backbench bullies from the other parties, had most to gain and duly gained it. "We need to be clear with you and straight with you" may sound like political blah but voters like honesty.

This was a huge evening for the Liberal Democrats. Clegg was helped merely by being there. But he also had to prove he was worthy of his equality with Brown and Cameron. There's not much doubt that he succeeded. No one event on its own is going provide the Liberal Democrats with the quantum boost they crave in order to be on level terms with the other parties. You don't become a credible prime minister overnight. But Clegg, who has often been damned with faint praise as a rather disappointing party leader since he succeeded Menzies Campbell, rose to the big occasion.

His final pitch was significant. Yes, he said, there is an alternative. It's not true that the parties are all the same. There's another option which Labour and the Tories will never give you. Liberal Democrats always say that, of course. But here was a Lib Dem leader saying it to a volatile electorate at a moment when, if he makes the sale, could change a lot of assumptions in the 2010 contest.

The initial polling looked good for the Lib Dems. The party would be gutted if it was anything else. But it will be some days before the dust – political as well as Icelandic – settles. Even so, I think this performance by Clegg will change the campaign. The other two parties know they have a dangerous enemy out there.

They are faced with a choice between ganging up against the Lib Dems or trying to be the Lib Dems' best friend. The Tories will certainly choose the former course because they are more at risk from a boost to Lib Dem fortunes than Labour is. Labour will be tempted to cuddle up to Clegg a bit, partly because a lot of Labour people think it's right to do so, partly because they think Clegg may be the key to keeping them in office. In the end, though, Labour can't afford to be too friendly.

So stand by for the two large parties to take the gloves off on everything from tax to immigration to Trident. Jo Grimond once asked the old Liberal party to march towards the sound of the gunfire. Nick Clegg's party can expect the gunfire to head their way. Things will get rough for them now. But a concerted turn against Clegg will play into his narrative of the way the other parties behave – making the same promises and breaking the same promises, as he put it. It is hard to predict the impact. But this could be exciting.

Cameron was the evening's disappointment. He also had the hardest hand to play. He looked less at ease and some watchers thought he talked down to them. Never underestimate the Conservative leader. He may have been kicking himself after last night, but the Tory party is a formidable machine, well funded, with important media allies and ruthless. Don't underestimate, either, how issues like immigration and nuclear defence play to Tory strengths with the public, whatever the liberal consensus may be. But expect to see the first "Panic at Tory HQ" and "Knives out for Cameron" stories this weekend.

Brown did OK. His people had been good at massaging the expectations. He went into the debate as the avowed underdog – it's the reason he agreed to take part in the first place – and he did solidly well. The Labour leader will have surprised those who thought he was simply a grumpy old political boss. Yes, he produced too many lists. Yes, he has not been able to make good on his claim to be "in the future business". But Brown played to his strengths last night. He sounded competent and experienced. He was right, from a tactical point of view, to attack Cameron, who remains unquestionably the Labour party's largest threat.

Even if the debate does not decisively shape the result on May 6, it may nevertheless help to boost the number of people who vote. This was both a large shared political event and most people's first exposure to the 2010 campaign. If today the debate turns into what Americans call a water-cooler moment – a subject that people discuss in the office – it could give turnout the single biggest kick up the backside it is likely to get in the coming weeks. In this context, forget Twitter. What matters, as researchers on both sides of the Atlantic have found, is old-fashioned local campaigning, talking face-to-face, in the home and on the doorstep.

The two things most likely to get people out of their homes and down to the polling station nowadays are a close race and the belief that there is a sharp divide between the main parties. By those yardsticks, 2010 is doing its best to oblige and the debate last night undoubtedly helped. The result on May 6 is more uncertain now than it was before and the argument over the role of the state and the role of society is a large one. If nothing else, the debate did credit to our much maligned politics while confirming that our political parties also have something serious to argue about. Roll on the next debate. This election has come alive at last.

This column was updated at 11.05pm to reflect revisions for late editions