Why David Cameron has already won the TV debate

Before the guests have arrived or the lights switched on, David Cameron has already won tonight's TV debate. Here's why:

It will have a minimal effect on the campaign

Cameron got the debate he wanted, not just in terms of who was invited, but when it took place. The broadcasters blinked first. The debate takes place broadly when Downing Street wanted it: before Easter. The Tories don't believe anyone is really paying attention to the election and won't be until after the Easter break. The event has been tabled at a time when there is least public demand for it. It is unlikely to get the 10 million viewers won by the debates in 2010. Anything above the 3 million scored for last week's Paxman grilling will be considered an achievement.

The timing also gives the Tories plenty of time to subdue any unexpected effects from the debate. Say Nigel Farage turns in a barn-storming performance, or Miliband does a Cleggmania. Even in these worst-case scenarios Cameron still has five weeks to enjoy the expected incumbency bounce ahead of polling day. Clegg-mania mostly dissipated by election day in 2010 and any unexpected spike in an opponent's support might be expected to do the same before May 7th.The timing of the debate has been precision engineered to reduce the electoral impact on the Conservatives.

Democratic bore-a-thon

Cameron banked on the idea that having a wide range of voices all bickering over each other for two hours would play to his advantage. By having so many parties on stage – many of whom can only be voted for in a minority of seats – he can stand off to the side pointing at them and saying: 'Do you want this rabble fighting among themselves the day after the election or would you rather have the firm hand of Conservative government?' It plays directly into the image Cameron wants to project ahead of the election.

The format of the debate encourages this. Leaders have a minute uninterrupted on each of the four topics – the NHS, immigration, the economy and the future of the UK – but after that it turns into a free-for-all. The prime minister will be hoping that the smaller parties speak over each other and Ed Miliband in a bid to make the most of their time in the spotlight, while he stands calmly offering a vision of certainty in a manic and bad-tempered political world.

Crowding the left

Cameron's biggest threat is Farage, who can express a robust right-wing message with less compromise and more conviction than the prime minister can. This is a core vote election. The two main parties are oscillating on roughly 33% and have given up trying to appeal to wavering centrist voters. Any inspiring appeals to the values of their core supporters from a smaller, more radical party could be the death knell of their campaign.

But if Farage presents a threat to Cameron, Miliband has three times the problem. On the left, the Labour leader is surrounded by Leanne Wood of Plaid Cymru, who was once arrested for protesting over Trident and told to leave the Welsh Assembly chamber for calling the Queen 'Mrs Windsor'; Nicola Sturgeon of the SNP, who is a fiery and accomplished debater intent on savaging Miliband for signing up to the austerity agenda; and Natalie Bennett of the Greens, who can promote precisely the sorts of policies core Labour supporters are desperate to see.

Cameron's less-than-convincing appeals for fairness for the Greens were nothing to do with democratic debate and everything to do with splitting his opponent's vote in the way Ukip splits his.

Avoiding Miliband head-to-head

Polling suggests Cameron has two main strengths: management of the economy and looking prime ministerial. The latter quality is deeply overstated. Under pressure of scrutiny, Cameron frequently wilts, as he did against Paxman last week. And the weaknesses of his opponent are also typically overstated, as was again evident against Paxman.  While the prime minister performs well against Miliband with the deafening jeers of the Tory benches behind him during PMQs, the head-to-head option, which rewards close attention to policy and a consistent track record, would have been far more dangerous.

While not exactly a latter-day Kennedy, Miliband is also much more talented than his detractors give him credit for. He seems more passionate and likeable than Cameron, as he demonstrated in an Absolute Radio interview this morning.

Cameron has made sure Miliband's voice will only be heard where it can be drowned out by all the other parties. He has done whatever he can to avoid being subject to the Labour leader's questioning. This may have robbed the public of the chance to measure up the two men who could be prime minister, but it was strategically sensible.

Following last week's Paxman event, Miliband has enjoyed much-improved polling for his personal qualities. When people see him – above the heads of the Tory-supporting press which is so desperate to mock him – they tend to like him. That's why his team were desperate to make the debates happen. It offered them uninterrupted access to the public without the press getting in the middle and bringing up a photo of him trying to eat a bacon sandwich.

He didn't get it. The broadcasters blinked first. Downing Street won. And no matter what happens in today's debate, it has been framed in every way to play to the advantages of the Tory leader.