What's At Stake For The Seven Party Leaders?

By By Jon Craig, Chief Political Correspondent

Tonight's TV debate is the big one, in terms of the number of party leaders taking part.

Seven leaders, including an initially-reluctant David Cameron, will clash for two hours in a debate chaired by ITV's Julie Etchingham, formerly of Sky News.

After Mr Cameron and Ed Miliband endured bruising interrogations by Jeremy Paxman in the Sky News/Channel 4 showdown, this time it's different: a real debate between the seven.

Still to come is a debate involving the leaders of the five smaller parties and then a Question Time style event with Mr Cameron, Mr Miliband and Nick Clegg.

But tonight is the only opportunity for the PM, Leader of the Opposition and DPM to challenge each other in debate, and perhaps also to be embarrassed by the leaders of the smaller parties.

So what do the seven stand to gain or lose?

:: David Cameron (Conservative)

Gain: The PM surely can't be as ill-prepared and exposed as he was by Jeremy Paxman's blistering opening questions on food banks and zero-hours contracts. He is likely to be better-briefed on the sort of questions that could leave him open to the charge that he's out of touch with ordinary voters.

Lose: After notoriously shouting "Calm down, dear!" across the despatch box at Labour's Angela Eagle in PMQs, he needs to be careful how he deals with his three female opponents, particularly the formidable Nicola Sturgeon of the SNP. Any "Bullingdon Club" or "Flashman" temper would be disastrous on TV.

:: Ed Miliband (Labour)

Gain: He needs to build on his "Hell yes!" toughness that began to emerge towards the end of his Paxman clash. At the same time, if he can appear more statesmanlike and more measured than the leaders of the smaller parties he may convince the many doubters among voters that he could be PM material after all.

Lose: If he's poor on the economy he'll be in big trouble. He will be attacked as a spendthrift socialist by Cameron and Clegg, but denounced for backing Tory cuts by Sturgeon and the Greens' Natalie Bennett. A member of the audience is bound to ask about his brother again. He was wobbly on that, too, first time.

:: Nick Clegg (Liberal Democrat)

Gain: This is his big chance, after being shut out of the Paxman interviews. He has a chance to claim the credit for Coalition successes like raising income tax thresholds. He can also sound moderate on the economy by attacking Tory cuts and Labour spending. He will have allies in the debate, too, for his pro-EU stance.

Lose: The same pro-EU stance will leave him vulnerable to attacks from UKIP's Nigel Farage, who roughed him up in last year's Euro-election debates. He also risks looking like an also-ran by being lumped together with the leaders of the minor parties. He will be accused of betrayal over his tuition fees U-turn too.

:: Nigel Farage (United Kingdom Independence Party)

Gain: A great opportunity to display his man-of-the-people common touch and kick lumps out of the established parties and portray them as "the Westminster elite". His humour and flamboyant style should come over well in a TV debate and if he's on form he could make his opponents look wooden and aloof from voters.

Lose: At the same time, his style alienates some voters, who regard him as "the pub bore" who's anti-everything and wants Britain to return to life as it was in the 1950s. He's vulnerable, too, on accusations against some Kippers of expense-fiddling, racism and homophobia. Needs to be careful debating against women opponents.

:: Nicola Sturgeon (Scottish National Party)

Gain: Scotland's new First Minister has the potential to be the star of the show and inflict more misery on Labour. She's more serious and business-like than the flamboyant Alex Salmond. She will attack the two big parties on austerity and Trident and David Cameron would be foolish to under-estimate her.

Lose: If the SNP has a weakness, it's that it can be guilty of complacency, though that was a fault of Salmond rather than Sturgeon. Ed Miliband will want to use the debate to fight back against the SNP. Critics also claim that on the economy, oil prices and the future of sterling the SNP's sums don't add up.

:: Natalie Bennett (Green Party)

Gain: For the Greens, this is the sort of publicity the party could only have dreamed of, until David Cameron - in his determination to damage Ed Miliband and Labour - insisted on them being invited to the party. It's a great chance to spell out the Green Party's policies and pinch votes off the Lib Dems as well.

Lose: Ms Bennett will need to be better briefed than when she self-destructed in a radio interview with LBC's Nick Ferrari, blaming a cold. She will be attacked by her opponents over the Greens' record in local government, particularly in Brighton. And many viewers may wonder why she's leader and not Caroline Lucas.

:: Leanne Wood (Plaid Cymru)

Gain: Just being included is already a gain for Plaid. Both they and the SNP currently have fewer MPs than the Democratic Unionist Party, who weren't invited. It's great publicity for a party that doesn't get much media attention UK-wide and has had nothing like the success of the SNP in Scotland.

Lose: Short of a Natalie Bennett style disaster, Plaid really has nothing to lose. They're there at the table with the big boys and girls, sharing a platform with a PM, a DPM and a Leader of the Opposition - who will be her harshest critics, for obvious reasons - and a First Minister whose poll ratings and popularity are to die for.

:: Watch the seven-way leaders' debate live and in full from 8pm on Sky News, on Sky channel 501, Virgin Media channel 602, Freeview channel 132, Freesat channel 202, and on the Sky News website.