Party political leaders will aim to neutralise the negatives

The ITV's leaders' debate porgramme will start at 8pm tonight. Credit: ITV News

Now that we are almost upon it, you can see exactly why the Tories weren't all that keen (colossal understatement) on a TV debate.

There is an unpredictable dynamic to it - a potential for drama - that is bound to make an incumbent Prime Minister nervous, especially when he is (probably, just about) ahead in the polls.

Many words will be spoken and written of tonight, but when all the fuss has died down the real winner will be the one who makes the most progress in his or her overall strategic mission.

If that sounds like gobbledygook, let me try to explain by looking at the issues confronting each leader in turn.

If you have followed politics at all these past few years, you'll know Nigel Farage has been very effectively riding a powerful protest wave. But since his party won almost thirty percent of the vote at the euro elections in 2014, the squeeze has really been on.

We know from the polls that many of UKIP's supporters would rather see David Cameron as Prime Minister than Ed Miliband and a lot of experts expect some of them at least to head back to the Tories on polling day itself.

So Nigel Farage's best hope is to convince his existing supporters that there is going to be a coalition or minority government and that, if he can get four or five MPs, he will have a very significant role to play.

The Green Party's Natalie Bennett will speak first and David Cameron will be last. Credit: Chris Radburn / PA WIRE

The same is true of Natalie Bennett and Leanne Wood, both of whom are newcomers to this kind of national exposure and will benefit enormously from an even modestly competent performance.

Anything better and they are going to have a very good night.

But again, they have to 'legitimise' a vote for a minor party and the way to do it is to convince people a minority government of one type or other is a given.

Otherwise, it is perfectly plausible for people to tell pollsters that they are all for Nigel or Natalie or Leanne - and then to go and vote for one of the other party leaders on the day.

David Cameron has a relatively simple task.

He is already ahead of his principal rival on the economy and leadership, but he has to convince people that there is nothing mean or ideological about his desire to go on cutting spending.

In one sense, his job tonight is to reassure voters who are less convinced than they were last time of the necessity of bearing down on the deficit.

The party leader's will be focusing on how to get their message across. Credit: PA

Nicola Sturgeon and Ed Miliband arguably have the trickiest task.

Nicola because she is known to be an excellent debater, expectations are therefore high and because her party is enjoying an incredible surge in the polls and any misstep might let Labour more convincingly back into the fight in Scotland, and Ed because he has to demonstrate to people that he has what it takes to be Prime Minister in a two-hour window, whilst simultaneously beating back attacks from Nicola and from David Cameron.

On the plus side, he is an intelligent man who is starting from a low base in terms of personal ratings so he can arguably only improve his standing.

The relevant question will not be who won and who lost - though winning that battle won't hurt - but who neutralised their negatives most effectively.

That is a very complex matrix to assess, so do join me on The Agenda after News at Ten tonight for what I hope will be a really interesting and insightful post match discussion.

I'll have Rory Bremner with me, the Editor of the Spectator Fraser Nelson, the world's best known TV correspondent Christiane Amanpour of CNN, and the critic and writer Grace Dent.

The Agenda with Tom Bradby is on ITV tonight at 10:45pm.

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