John Rentoul: How many years out in the cold?

If David Cameron wins the the election in May, the big question is this: will he be prime minister for four years or for eight? This is getting ahead of ourselves, I know. We do not want to take the great British public for granted. But let us take the great British public for granted, and assume that they will not warm to Gordon Brown despite his recent rediscovery of a public sense of humour. The probability of Cameron's becoming prime minister implied by the bookmakers' odds is about 90 per cent. This is because, even if the Tories fail to win a majority, they are likely to be largest party and therefore have the better claim to form a minority government.

How the election campaign will be fought is interesting and all that, and might make a small difference to the result, but the important question is: what next? Because there is a big difference between Labour's being out of office for four years or for eight. A Cameron government, elected on a programme of protecting the NHS, promoting new non-selective state schools and furthering equality, cannot do much to dismantle the Blair-Brown social democratic settlement in a single parliament.

Yes, Cameron and George Osborne (who gave a rare interview to Newsnight last week) want to cut public spending earlier and more deeply than Labour does. But the difference is being pared and sanded down all the time. By polling day, it will be reduced to a notional cut in the fiscal year beginning in April, and the smallest possible extra cut on top of Labour's plans thereafter that is visible to the naked eye. Like the plan to "recognise marriage in the tax system", now reduced to an unspecified but nugatory amount, that Cameron candidly admitted was "about the message more than the money", the supposedly harsher spending cuts are largely symbolic. Now symbolism is important in politics, but it takes a long time to work.

Thus it will not be until the election after next that the Tory party will be able to return to the small-state, tax-cutting place where its heart is. Not that taxes are likely to be cuttable even then, given the disastrous state of the public finances, but by then tax cuts may figure as part of the Tories' "forward offer", or whatever the heirs of Steve Hilton call it.

That, then, is why it makes a difference whether Cameron is a two-term prime minister or not. If he lasts only four years, a revived Labour Party could resume the social democratic march that has been temporarily halted; if he makes it eight, Britain may look like a Conservative country again.

So what is likely to happen? That depends on how the main parties handle their internal divisions, in the new contexts of office and opposition.

The Tories have been fortunate in that their internal tensions have attracted little attention since the Great Grammar School Row of 2007. But that will not be so easy in government. Our ComRes poll today suggests a Tory majority of about 70. Brown, with a majority about that size, has still let rebels drive policy – remember Post Office privatisation? Neither do I. It disappeared from the legislative programme when Labour backbenchers flexed their muscles. Blair, with a majority more than twice as great, managed to win the critical vote on tuition fees by a margin of just five. On issues about which Tory MPs feel strongly – Europe, climate change, grammar schools – Cameron may be forced to trim. Above all, he faces the challenge of putting up taxes and cutting spending, and the party is bound to be torn between those that want the emphasis more on one than on the other.

The new intake of Tory MPs is conspicuous not for being a clone army of Cameroons, but for a flotilla of mavericks (Rory Stewart, Zac Goldsmith) in a sea of traditional Tories. David Davis, the former shadow home secretary who quit to tilt at the windmill of the Labour police state, is still hungrily prowling the corridors of Westminster. Outside the stockade, Boris Johnson's star is still rising, with an independent power base that in France would be regarded as a stepping stone to the premiership. Davis mutters about grammar schools; Johnson splutters about taxing bankers.

Labour faces a different challenge. Will it respond to defeat as it did in 1979, by going left? Or as the Tories did in 1997, by mouthing the platitudes of modernisation before running back to their core vote? Or will it adopt the Domino's Pizza strategy? Domino's in the US ran adverts last month featuring customers saying its products tasted "like cardboard", and saying that it had listened, learned and changed its recipe. That would require the party to keep the recriminations short. The Blairites will blame the Brownites for crashing their car; the Brownites will blame the Blairites for cutting the brake cables. But then they need to unite and move on to different policies for different times.

There are hopeful signs for Labour. As our political editor reported last Sunday, Jon Cruddas, the key figure on the left, is "increasingly likely to stand" for the leadership. That in itself is interesting, for Cruddas is a thoughtful and unifying leftwinger. But there is more. I am told that he is also open to the possibility of running as David Miliband's deputy.

Miliband is the bookies' favourite for next Labour leader, and is building alliances across the party, from James Purnell and Cruddas to Harriet Harman. (The really intriguing question about this month's attempted coup against Brown is what Harman was up to: did she want Brown's job for herself or for her unexpected new ally, the Foreign Secretary?)

Looking to the future always tempts wise heads to murmur about "events". But the impact of unexpected events is less important than how parties and party leaders respond to them. So this is the question that Labour ought to ask itself if it finds itself out of office on 7 May: who would be most likely to bring the party together and lead it back to power in 2014?

John Rentoul blogs at independent.co.uk/eagleeye