Here comes the leaders' debate

It might be overselling it say the 7-way leaders' debate this evening is hotly anticipated, but the second televised election set-piece at least offers us the opportunity to watch the party heads take each other on.

We'll be covering every cut and thrust live but here's a brief crib sheet ahead of the kick-off.

Although the prime minister narrowly shaded the first of head-to-head against his Labour opponent, for the two largest parties the ambition will be to escape this evening relatively unscathed. Labour and the Tories are effectively neck-and-neck in the polls and neither will want to take too many risks with just over a month to go before the General Election.

However, standing in their way are a raft of smaller parties each intent on making their mark by scoring points at the expense of David Cameron and Ed Miliband.

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I would expect some strongly-worded barbs from bellicose UKIP leader Nigel Farage, while the SNP's Nicola Sturgeon will equally want a stand-out performance to lock in her lead in Scotland and pull herself out from former nationalist party leader Alex Salmond's long shadow.

It will likely be a bittersweet evening for the Lib Dems' Nick Clegg, the clear winner of the 2010 head-to-heads. With his party's support less than half what it was then he'll at least want to perform better on stage than he seems to be doing in his local polls.

Meanwhile, Green leader Natalie Bennett will be hoping to shrug off her recent car-crash of an interview with LBC in which she struggled to recall her own party's policies (she later blamed her poor performance on a cold) and revitalise the Green surge that looks increasingly to be running out of steam.

And Plaid Cymru? Well, they'll probably just be pleased with getting onto a national platform.

(Some are also asking the question - where oh where is the DUP, the UK's fourth largest party by parliamentary seats, in all this?)

Whatever the faults of the format (and they are legion), the risks for Miliband and Cameron are still clear.

If Labour are to stand any hope of forming a government they will need to get voters believing that Ed is, in his own words, "tough enough" to lead a parliamentary group that may have to include SNP votes. He says "hell yes" but the public may need a little more convincing.

Cameron is hoping that the painful negotiations that he put the broadcasters through over these election debates have won him exactly what he wanted - a messy platform of bickering parties that he can rise above prime-ministerially. He's even put himself on the end of the row so that he can get the last word.

If Farage manages to score a few telling blows against him in the debate, however, that strategy could unravel. And quickly.

The smaller parties undoubtedly have the most to gain from a strong performance, but equally the most to lose as most of their poll gains have come on the back of people switching parties - and a weak performance might just encourage them to revert to one of the mainstream parties. They are, however, the wildcard of the evening.

Oh, and we're all waiting to see whether #IagreewithNicola becomes the catchphrase of the 2015 debates much like #IagreewithNick was in 2010.

We'll be covering all the action live. Click here for updates.

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