Election 2015 TV debate: And the winner is…

With the polls pointing towards another hung parliament, the leaders have all to play for

Who won? That is the question everyone will ask at 10pm tonight, so before anyone answers perhaps we should pause and ask something else: what on earth does winning a TV debate really mean? And how will we know?

Should we take seriously the instant polls of the few who choose to watch all two hours of seven party leaders arguing on the eve of the Easter holidays? Maybe not.

They are likely to be a small and pretty unrepresentative sample of voters as a whole. Not only that. They will include many whose minds were made up long ago. And the inclusion of the Scottish and Welsh nationalists means that most viewers will not be able to cast a vote for two of the leaders they are watching.

So, how about the verdict of Twitter or Facebook? Ditto. Social media lends a loudhailer to the committed, the partisan and organised.

What then about the sage words of the commentariat as they emerge from the hothouse of the "spin room"? Cramming political journalists, spin doctors and live cameras into a room is the best possible guarantee that the media "pack" will produce its own, not particularly reliable, conventional wisdom.

The 'undecideds'

What really matters is the judgement of those who have yet to decide how to vote - but we cannot ignore all of the above for one fundamental reason. Most voters - perhaps particularly the "undecideds" - will come to a view of who won without actually having watched any of the debate at all. Many others will have watched for just a few minutes.

That is why the spin doctors try so hard to establish that their leader came out on top. That is why the selection of the clips that make it onto news broadcasts or onto You Tube or Facebook are so important.

The podiums await for the seven party leaders, each of whom will be questioned on four topics

Ed Miliband and David Cameron faced questions from a live audience last week

The Conservative and Labour leaders were grilled separately by Jeremy Paxman

Look what happened after last week's debate that was not really a debate - the Paxman interviews with David Cameron and Ed Miliband.

First an ICM poll had the PM winning*. Then along came a Sunday Times YouGov poll that was used by the paper to suggest Mr Miliband's victory was so clear it had broken the deadlock between the two parties. Days later other polls suggested nothing much had changed at all.

Memorable moments

My view, for what it is worth, is that Ed did win both because he defied expectations - he could hardly fail to - and because he provided the best memorable "moment" of the night.

The Labour leader has always been a much better performer than many people realise. The candid view in his office is that many voters who see him for a minute or less think he is hopeless but if they watch him for longer they start to change their minds. That is why he was so desperate for a head-to-head debate with Mr Cameron and why the prime minister was equally keen to avoid one.

Both men were left squirming by the Paxo inquisition - Cameron on zero hours and foodbanks; Miliband on immigration and borrowing. But the Labour leader's defiant assertion that "hell, yes" he was tough enough for the job was a much more memorable "moment" and got much more airtime.

Tonight's programme will, of course, be different. Not just because it is an actual debate but because seven leaders are taking part.

Challenging preconceptions

Four of them will have won before a word has been uttered. UKIP's Nigel Farage and the late additions - Natalie Bennett of the Greens, Nicola Sturgeon of the SNP and Leanne Wood of Plaid - have been given equal billing with the prime minister, deputy prime minister and leader of the Opposition. It is a status they and their predecessors have craved for decades and always been denied.

What is more, tonight will, in reality, be a series of mini debates - the big one between the two candidates for prime minister, plus a battle on the right between Mr Cameron and Mr Farage; one on the left between Mr Miliband, Ms Bennett, Ms Sturgeon in Scotland and Ms Wood in Wales and one in the centre in which Nick Clegg will try to show that five more years with him in office would be better than allowing either the Tories or Labour to govern alone.

Observing this from the sidelines** I sense an election campaign that has yet to come to life and reporters who are craving excitement.

So, what will matter most about tonight's debate is if it changes the narrative - as did the "I agree with Nick" debate which produced Cleggmania five years ago, or in the way Alastair Darling's surprise mauling of Alex Salmond did in the first debate on Scottish independence.

That is why I suspect all leaders will have learnt from David Cameron's performance last week that it is simply not enough to set out to calmly persuade those who have bothered to watch and not enough to avoid making any embarrassing gaffes. You also need to say something clippable and memorable that challenges voters' preconceptions of you and helps to write the next chapter of this election saga.

This, after all, is the only full-scale debate we will see.

* Here's a few more facts on last week's programme:

• The "undecideds" polled gave the win clearly to the Labour leader

• In YouGov's poll - done in the days after the debate - 60% of their respondents acknowledged that they had neither seen the programme nor any coverage of it despite it dominating Friday's news. Only around 7% watched the whole programme

• Ed Miliband's leadership ratings did increase in a number of recent polls but there is no way of showing there was a direct link with last Thursday's programme

• #BattleForNumber10 shot to the top of Twitter's list of UK and worldwide trends, with more than a quarter of a million posts. There were similarly big numbers under related hashtags

** I'm watching from the sidelines as I am recovering from an op and undergoing a course of chemotherapy.