Gordon Brown’s poor performance in the last weeks of 2007 perplexed many observers. How could a politician who for decades has played at the top table fumble so many issues?

One answer is that he has the wrong kind of experience. He is highly unusual for having held just one ministerial job before prime minister, that of chancellor of the exchequer. To have held that office alone is the worst possible preparation for the premiership.

A chancellor does well to keep aloof from the consequences “on the ground” of his fiscal decisions. He does not want his judgment of the right level of tax or spending to be clouded by emotional arguments presented by nurses, teachers or pensioners. The chancellor decrees policy, and other departmental ministers are left to deal with what the Treasury calls the “bleeding stumps” - the cataclysmic outcomes regularly predicted by lobby groups.

Other chancellors who became prime minister - such as James Callaghan and John Major - had an advantage over Brown. They had spent years as junior ministers, becoming adept at handling vested interests and at judging how far they exaggerated the impact of policy changes.

In preparing his budget the chancellor traditionally has a unique degree of autonomy. But few can, like Brown, have refused to consult even the prime minister. Unlike most chancellors, who must deal with other cabinet ministers as colleagues, Brown was essentially in command of them, so he was not obliged even to hear their arguments.

He took detachment to extremes, to judge by the recent testimony from Lord Guthrie, a former chief of the defence staff, who said Brown refused to visit the defence ministry to be briefed on military campaigns. That was part of his McCavity ploy: to remain ignorant of issues, notably foreign policy, that were unpopular and on which Tony Blair excelled.

Before he entered No 10, whenever Brown addressed parliament it was on his terms. By convention no MP interrupts the budget speech. The chancellor faces questions only once a month when the house is sitting, unlike the prime minister, who is grilled weekly. Questions to the chancellor are specific, not general like those addressed to the premier.

Brown could choose which topics to field himself and which to farm out to junior Treasury ministers. If he felt like handling only an arcane matter such as Vat or capital allowances he could do so. House rules confined all follow-up questions to that specific subject.

So Brown has little experience of listening to colleagues, needing to win an argument, engaging in unpopular policy decisions or wrestling with an unfettered Commons.

If he had been party to the painful decisions on Iraq, perhaps he would have been decisive about whether to call an election last autumn. Had he been less aloof, maybe he would have known about the Labour party’s funding scandals and addressed them. With more diverse experience of parliament, he might have developed those self-deprecatory swerves that so often rescued Blair, winning him appreciation even on his darkest days.

If Brown is by nature impatient, nervous, bad-tempered and suspicious, nothing in his political education has helped to smooth the edges. He has developed no camouflages and surprisingly few techniques. There is little sign that he is learning on the job.

In Canada, Paul Martin’s career bore an extraordinary resemblance to Brown’s. He served for 10 years as minister of finance, having held no other government position. His tenure was punctuated by serious disagreements with his prime minister, Jean Chrétien, with Martin campaigning to replace him. Eventually he did. Having inherited a strong parliamentary position, after six months Martin lost his majority and 18 months later was defeated at the polls. The Conservatives supplanted him with a minority government.

Given that Brown has ruled out an election this year, David Cameron cannot hope to dispatch him quite so quickly. But he can play on the prime minister’s shortcomings. Once, Cameron must have feared that Brown’s experience would give him a crucial edge. Not necessarily.

We saw what 10 years in power did to Margaret Thatcher and Blair. It wore them down and, more important, entrenched their worst habits. They became increasingly doctrinaire and impatient of dissent. For Brown it is hard to look fresh after a decade of applying the Presbyterian work ethic to micromanaging the economy. He is unlikely suddenly to become collegiate after years of autocracy.

To make matters worse, he has appointed a weak cabinet, a sign of weakness in itself. There is no minister who would say boo to a goose, nobody to match David Blunkett, Charles Clarke or John Reid. Brown hardly looks taller by surrounding himself with short grasses.

Thatcher and Blair used foreign policy to dwarf their political opponents.

Once she had been lionised by an admiring crowd in the Soviet Union her domestic opponent, Neil Kinnock, looked a pygmy.

Iraq made Blair unpopular, but he was unquestionably a towering figure on the world stage. Brown, however, has looked tentative in dealing with America. Nor has he been sure-footed on Iraq and Afghanistan. He is failing to exploit a key advantage of incumbency.

As a sign of his growing confidence, Cameron launched the new political season with an attempt to make the Conservatives “the party of the National Health Service”. After last October’s proposals to cut inheritance tax, Cameron is reasserting that his main strategy is to occupy Labour territory rather than reinforce ground that the Tories hold already. It is, intentionally, reminiscent of Blair’s seizure in the 1990s of traditional Conservative banners such as law and order.

Yet while the NHS is a natural issue for Labour, his prescription is recognisably Tory. He wants to dump targets and centralised control, methods particularly associated with Brown.

Cameron may be too plummy for everyone’s taste but at least he is affable. For all the criticism that he is excessively like Blair, people simply prefer someone cheerful and charming to someone grouchy and defensive. The standing ovation Blair received on his last appearance in the Commons underlines the point. Many MPs could not help liking him despite everything. The media need a show, and Cameron, a man who can hug a husky, is a better showman than Brown.

It seems unlikely that the next election will be decided on policy. The differences between the parties on health and education are academic. In any case, like poker players, the parties adjust their proposals to match their opponents’ last bid.

So it must be decided on character instead. For example, whether or not the government succeeds in changing the time limit for holding terror suspects without charge, our question on polling day should be: do we trust Brown or Cameron more? In the uncertain years ahead, our next prime minister may have to choose between peace and war, and balance civil liberties against powers for the state.

If Cameron can debunk Brown’s reputation for experience, showing it to be a liability not an asset, then he can work the character issue to his advantage. He will implant the question: does our prime minister have the temperament to decide the big issues? Is his judgment flawed?

Even if Cameron succeeds, I give the Conservatives little chance of winning. They have not consistently polled enough to put them within reach of a majority. They have fewer seats after 10 years in opposition than Michael Foot won at the 1983 election. Cameron has not found an “aspiration” issue to attract new supporters, as Thatcher did with her promise to allow council tenants to buy their homes.

Maybe Cameron pins his hopes on the Canadian example. The Progressive Conservatives were reduced to just two seats in the election of 1993. By 2006 a new Conservative party was back in office. True enough, the turnaround he needs is small by comparison.

Martin Ivens is away

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