In the final part of his series examining the state of the major parties Richard Porritt gives his view of the Conservatives

David Cameron’s Conservatives face a watershed moment.

When earlier this week South Suffolk MP Tim Yeo stuck his head above the parapet and asked whether the PM would be a “man or a mouse” over a potential U-turn on a new runway at Heathrow he was simply vocalising publicly what many backbenchers had been whispering in Westminster’s corridors.

Number 10 and Coalition partner Nick Clegg were quick to quash the rumours of a rethink over this powder-keg issue but the feeling remains that Mr Cameron and the Tories need to be brave. They need to take more risks.

More than two years into power the Conservatives are having a tough time. The Olympics provided some respite but the facts are that Britain remains in the doldrums and people are getting fed up of words without the actions to back them up.

This year has seen a rumbling unrest within the Tory ranks – backbenchers fear, through no fault of their own, their chances of holding on to marginal seats are being scuppered by national policies. Often it matters little how hard an MP works in Westminster – and the vast majority, contrary to popular belief, work extremely hard – or how many hours they spent listening to every little grumble of every constituent: If the party screws up on a national scale they face the boot.

So, growing concern is perfectly natural.

To be fair to the Tories they were very open with the public about how hard the cuts would be and they remain honest about how bitter the pills the country still has to swallow will be. But bad news followed by worse news is not the stuff election victories are made of.

Too often the Tories point the finger but the cries of “it would have been so much worse if Labour were still in power” or “if it had not been for that lot we would not be in the mess” are increasingly falling on deaf ears. It is possibly British politics’ worst trait – Gordon Brown was still attacking the previous Tory administration for the country’s woes at Prime Minister’s Questions just months before he was dumped out of power himself. The public does not want to hear form the ruling party about “what ifs”. Hypothetical politics is pathetic politics.

The Tories would do better completely ignoring Labour, let them play opposition while they govern. The man on the street wants the plan laid out in bullet points, he wants leadership. The Tories should still be trying to win votes like they did when they were in opposition rather than guard the ones they did get – not enough to actually win the election – and hope to poison swing-voters’ minds.

So with conference season looming and a reshuffle of cabinet around the corner Mr Cameron needs to be brave. The Conservatives are still well placed to win the next election outright and that is what is desperately needed if they want to really shape the future of Britain in their vision. Talk of Vince Cable ousting George Osborne filled newspaper columns – including this one to some extent – during the summer but it will not happen. A few new junior faces and the return of David Laws should be enough to spice up the top table.

What the Conservatives really need as they face the long, hard slog towards the next general election – without even the Olympics to puncture the gloom – is impetus. They had it in 2010 but now it is gone. All the economic targets are too far away and seem to be forever getting put further back.

What Mr Cameron needs is a blockbuster with an attainable result at the end, and hopefully before 2015. And Heathrow’s third runway would have proved an obvious choice. It is not just about the potential economic boost it is about saying to the world “we will not be beaten, after the recession we will be even more ready than before”. There are downsides, the obvious one being any environmental impact, but these are desperate times and plodding along is not good enough.

But it appears Heathrow is too much for a U-turn for Mr Cameron – possibly wary of the fact he promised it would not get the go-ahead in the manifesto – so he needs something else. Economists from all sides of the argument agree what Britain needs now is growth. And most agree the best way to do this is an infrastructure project and airports – even if not Heathrow – fit the bill.

Now Mr Cameron will not like this but maybe Boris Johnson’s plan for an airport in the Thames Estuary is exactly what Britain needs? It does tick a lot of boxes and it would certainly send a firm message about this country’s ambitions.

Of course there will be opposition groups, there will be some backbenchers in his own party who turn against him and some votes will be lost. But he needs to take his chance. Being popular in opposition is easy, when you are in charge you need to be willing to take the risk of people hating you. And he is willing – the cuts have proved that.

The next general election could be all but sealed in the next year. These are vital times where popularity for the PM can be won or lost just as easily. He and his Chancellor have made it clear they are not going to spend, spend, spend but maybe a spate of build, build, build will breathe life in to the economy and even the chances of an all out Tory victory.

Richard Porritt is on Twitter @Porritt.